Fine PM measurements: personal and indoor air monitoring

Chemosphere 49 (2002) 993–1007 www.elsevier.com/locate/chemosphere

Fine PM measurements: personal and indoor air monitoring M. Jantunen

a,* ,

O. H€ anninen b, K. Koistinen b, J.H. Hashim

c

a

EC Joint Research Centre, Institute of the Environment, Air Quality Unit, TP 272, I-21020 Ispra (VA), Italy b KTL-Department of Environmental Hygiene, P.O. Box 95, FIN-70701 Kuopio, Finland c Universiti Kebangsaan Malaysia, Faculty of Medicine, Environmental Health Unit, Kuala Lumpur, Malaysia Received 22 November 2000; accepted 25 October 2001

Abstract This review compiles personal and indoor microenvironment particulate matter (PM) monitoring needs from recently set research objectives, most importantly the NRC published ‘‘Research Priorities for Airborne Particulate Matter (1998)’’. Techniques and equipment used to monitor PM personal exposures and microenvironment concentrations and the constituents of the sampled PM during the last 20 years are then reviewed. Development objectives are set and discussed for personal and microenvironment PM samplers and monitors, for filter materials, and analytical laboratory techniques for equipment calibration, filter weighing and laboratory climate control. The progress is leading towards smaller sample flows, lighter, silent, independent (battery powered) monitors with data logging capacity to store microenvironment or activity relevant sensor data, advanced flow controls and continuous recording of the concentration. The best filters are non-hygroscopic, chemically pure and inert, and physically robust against mechanical wear. Semiautomatic and primary standard equivalent positive displacement flow meters are replacing the less accurate methods in flow calibration, and also personal sampling flow rates should become mass flow controlled (with or without volumetric compensation for pressure and temperature changes). In the weighing laboratory the alternatives are climatic control (set temperature and relative humidity), and mechanically simpler thermostatic heating, air conditioning and dehumidification systems combined with numerical control of temperature, humidity and pressure effects on flow calibration and filter weighing.
2002 Elsevier Science Ltd. All rights reserved. Keywords: Particulate monitoring; Personal sampling; Indoor sampling; Exposure studies; Development needs

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.1. Particulate matter; TSP, RSP, RPM, PM10, PM2.5 . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.2. Why combustion-generated PM, and why personal and microenvironmental monitoring methods? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.2.1. Combustion-generated particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.2.2. Personal and microenvironment monitoring . . . . . . . . . . . . . . . . . . . . . . . . 994 1.3. Theoretical aspects of personal and microenvironment PM mass sampling . . . . . . . . . 995 1.4. History of personal and microenvironment PM sampling . . . . . . . . . . . . . . . . . . . . . 996 1.4.1. Europe and North America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996

*

Corresponding author. Fax: +39-358-17-201-184. E-mail address: matti.jantunen@ktl.fi (M. Jantunen).

0045-6535/02/$ - see front matter
2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 2 ) 0 0 2 7 2 - 2

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1.4.2. Developing world . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3. Other PM-metrics: BS, BC and particle numbers . . . . . . . . . . . . . . . . . . . . . 2. Present personal and microenvironment PM10 and PM2.5 samplers. . . . . . . . . . . . . . . . . . 3. Development needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Monitoring equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Filter media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Laboratory methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Flow calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Filter weighing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Climate control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. Particulate matter; TSP, RSP, RPM, PM10 , PM2:5 TSP is abbreviated from total suspended particulates as collected by the standard high volume (HiVol) sampler; RSP and RPM are abbreviated from respirable suspended particles and respirable particulate matter (PM), usually referring to particles smaller than 3–4.5 lm in aerodynamic diameter, and mostly applied in industrial hygiene measurements. PM10 , PM3:5 or PM2:5 refer to PM with aerodynamic diameter smaller than 10, 3.5 or 2.5 lm respectively. These size limits are not sharp; the cyclone and impactor pre-separators remove half of the particles at the cut size and larger particles with increasing efficiency. 1.2. Why combustion-generated PM, and why personal and microenvironmental monitoring methods? Several recent scientific reviews of the health effects of PM and evaluations of uncertainties in our knowledge have produced priority lists of research needs (e.g. center of disease control (CDC) according to Lippman et al., 1996; EPA, 1996a, 1998; CASAC, 1997; NRC, 1998). National Research Council (NRC, 1998) lists the following questions as having the highest priority: 1. What are the quantitative relationships between the ambient PM concentrations and actual individual exposures (Research Topic 1 in EPA, 1996a,b, 1998; CASAC, 1997; NRC, 1998). 2. What are the exposures to the specific constituents of PM that cause health responses in susceptible sub populations and in the general population (Research Topic 2 in Lippman et al., 1996; CASAC, 1997; EPA, 1998; NRC, 1998). 3. What kind of advanced mathematical and modeling tools can be developed to represent the relationships

999 999 1001 1002 1002 1003 1003 1003 1003 1004 1004 1005 1005

between specific sources of PM and human exposures? (Research Topic 3 in NRC, 1998). 4. How can these exposure analysis tools be applied to link the harmful PM constituents to their sources and to promote effective air quality management to protect human health (Research Topic 4 in EPA, 1996a,b; NRC, 1998).

1.2.1. Combustion-generated particles Recent analysis by Laden et al. (2000) of the ambient air PM2:5 sample constituents and their associations to observed mortality in the six cities study (Dockery et al., 1993) has indicated that combustion-generated constituents of fine PM are strongly associated with the increased mortality. The association is even stronger between health effects and traffic-generated particles. The crustal elements show no such association (Laden et al., 2000). Also particles generated in indoor combustion sources, tobacco smoke (Hackshaw et al., 1997; Law et al., 1997) and smoke from solid fuel burning (Mumford et al., 1987) have been shown to increase mortality and morbidity from lung cancer and other diseases. 1.2.2. Personal and microenvironment monitoring Any progress in the above listed three research priorities requires monitoring of personal exposures to PM and measurements of PM concentrations in those microenvironments where populations and individuals spend most of their time and/or acquire most of their exposure. Koutrakis and Lioy (1998) raise the issue about the relationship of personal exposures to ambient air PM monitoring at the s.c. EPA supersites. The need is to generate realistic information on the exposure patterns for compounds and size/mass fractions of concern in ambient air, and therefore, to develop monitors to determine the part of personal and microenvironment exposure that is derived from ambient PM and, better still,

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Table 1 Basic mass data for personal and microenvironment PM2:5 and PM10 sampling Flow rate (l/min)

Filter diam (mm)

Personal 2.0

25

70

37

100

25

70

37

100

4.0

Microenvironment 10.0 37

16.7

Tare mass (mg)

100

47

120

37

100

47

120

Concentration (lg/m3 )

10 50 10 50 10 50 10 50 10 50 10 50 10 50 10 50

12-h sampling time Smpl. vol. (m3 ) 1.44 1.44 1.44 1.44 2.88 2.88 2.88 2.88 7.2 7.2 7.2 7.2 12 12 12 12

48-h sampling time

Net mass (lg)

Net/tare (‰)

Smpl. vol. (m3 )

Net mass (lg)

Net/tare (‰)

14 72 14 72 29 144 29 144

0.2 1.0 0.1 0.7 0.4 2.1 0.3 1.4

5.76 5.76 5.76 5.76 11.52 11.52 11.52 11.52

58 288 58 288 115 576 115 576

0.8 4.1 0.6 2.9 1.6 8.2 1.2 5.8

72 360 72 360 120 600 120 600

0.7 3.6 0.6 3.0 1.2 6.0 1.0 5.0

28.8 28.8 28.8 28.8 48.01 48.01 48.01 48.01

288 1440 288 1440 480 2400 480 2400

2.9 14.4 2.4 12.0 4.8 24.0 4.0 20.0

Typical Teflon filters with support rings are assumed.

from the different sources. Wilson et al. (2000) suggest a method for dividing the exposure to PM10 and PM2:5 into two fractions; exposure to PM of ambient origin, which is correlated with ambient PM, and exposure to PM of non-ambient origin, which is uncorrelated with ambient PM. Oglesby et al. (2000) used elemental analyses of the EXPOLIS data from Basle, Switzerland, to analyze personal to ambient concentration relationships. They show that while total PM2:5 exposure levels correlate poorly with ambient air PM2:5 concentrations, this correlation is high for sulfur (secondary PM) and potassium (wood burning). The correlation is surprisingly weak for Pb and Br (traffic) and zero for Ca. 1.3. Theoretical aspects of personal and microenvironment PM mass sampling Development of new personal and microenvironment samplers is important for the success in fulfilling the research needs discussed listed above. The key requirement in especially personal sampling is sufficient sensitivity for collecting 12 h integrated PM2:5 and/or PM10 samples. The PM concentration in air is almost always determined from a small (net) difference between two much larger readings, the pre- (tare) and post-sampling (gross) filter weighing results. Table 1 presents sample to filter mass ratios for ranges of typical personal and microenvironment PM samples. In personal sampling the smallest masses are in the order of 10 lg. Consequently a

microbalance with 1 lg sensitivity is required and variations in weighing conditions must be controlled or accounted for. This is especially important for smaller particle size fraction samples like PM2:5 , which have typically only about 1=2 of the mass of a PM10 sample. Sample mass to filter mass ratios around 1=1000 are common, and consequently a filter weighing precision of 5 significant digits or better must be aimed at. Very few physical measurements reach such precision. Small changes––in the order of 0.01%––in the observed filter tare mass between the pre- and post-sample weighing may profoundly distort the net mass calculation. The causes, effects and some remedies of such distorting phenomena are discussed later in this paper. The low sample mass forms the primary boundary condition for elemental and chemical analyses of these samples. The very low sample mass to filter mass ratio forms the secondary boundary condition, i.e. sets the requirements for the integrity, stability and purity of the filter material and the control of contamination at every stage of filter handling. If 0.1% of a compound in the sample needs to be the level of detection (LOD) for this analysis, the maximum allowed contamination in the filter needs to be in the order of 0.1 ppm. A method that determines the PM concentration in air without a proportionally massive filter requires much smaller methodological precision to achieve similar precision in concentration. Such methods exist; e.g. particle counters, electrical and optical aerosol monitors and spectrometers and differential mobility analyzers. Of

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these methods only the last one analyses particles (almost directly) by mass. On the other hand only optical monitors are presently sufficiently small, light, cheap and robust for personal and microenvironment measurements. For a comprehensive overview of PM sampling and analysis methods in general, reader is referred to the review of Chow (1995), and for a comprehensive overview on indoor air (microenvironment) PM study results and methods to the review of Wallace (1996). Because of these existing reviews, the emphasis of the present paper is more on personal than microenvironment PM measurement methods. 1.4. History of personal and microenvironment PM sampling 1.4.1. Europe and North America Study design characteristics and the personal and microenvironment sampling equipment used in the studies discussed in this chapter are summarized in Table 2. In an early study on personal exposures to respirable PM (RPM), 37 volunteers in Watertown MA and Steubenville OH carried personal samplers and filled time activity/diaries 12 h at the time (Dockery and Spengler, 1981). The main result of this study was that the 12-h mean personal RPM exposure levels were in reasonably good agreement with the mean outdoor concentrations. Sexton et al. (1984) assessed personal RPM exposures of 48 volunteers in Waterbury Vermont. They carried personal samplers and pumps and filled time activity/diaries every other day for two weeks and their homes were also equipped with similar indoor and outdoor microenvironment RPM samplers. Their main finding was in contrast with the previous study; outdoor particle level was not an important determinant of personal exposure. Personal exposures levels were found to be higher than indoor air levels, which were higher than outdoor air levels. A total of 97 non-smoking volunteers in two rural Tennessee communities took part in the next personal RPM exposure measurement and modeling study (Spengler et al., 1985). Harvard/EPRI personal samplers were used with cyclone pre-separators that pass 50% of 3.5 lm particles and 0% of 10 lm particles. The volunteers carried the personal samplers, their homes were equipped with indoor microenvironment samplers and ambient levels were monitored using centrally located samplers in each of the towns. Personal exposures of ETS-exposed people were found to be twice those of the non-ETS-exposed. Lioy et al. (1990) used a new personal MS&T sharp cut PM10 impactor together with a 4 l/min personal pump in Phillisburg, NJ, to evaluate personal exposures of 14 non-smoking volunteers. Eight PM10 samplers

were used in indoor microenvironments, and 4 in outdoor environments, as a part of the total human environmental exposure study (THEES). The unique feature in the THEES study design is the fact that all the 14 volunteers were monitored with simultaneous sampling for 14 consecutive days and nights. This design allows analysis of both within and between individual variability. In the late Ô80Õs USEPA developed a new particletotal exposure assessment methodology (PTEAM) as continuation to the CO- and VOC-TEAM studies. PTEAM study estimated the frequency distribution of human exposures to aerosol particles. A new battery powered carry-on-belt sampling pump was developed capable of 4 l/min flow rate for 12 h through a new personal PM2:5 or PM10 impactor and filter pack. This new personal sampler (PEM) differed from the MS&T design by having six instead of one impactor jet and using a 37 mm instead of 25 mm filter. Both features reduced its flow resistance, and consequently air pump and battery requirements. This PEM and prototypes of a new AC-power driven microenvironment PM10 sampler (Marple et al., 1987) were tested both in the laboratory (Anderson et al., 1989) and in the field (Spengler et al., 1989; Buckley et al., 1991). The whole PTEAM study protocol was tested in a 9 home pilot study in San Gabriel, CA (Clayton et al., 1991). Field reliability, precision (comparison of duplicate samples) and repeatability (comparison to the average of two SSI (Standard Sampling Instruments)) of the devices were found to be good or acceptable (Wiener et al., 1990). The PTEAM study evaluated personal PM10 exposures and microenvironment PM10 and PM2:5 concentrations of the population of Riverside, CA (Wallace € zkaynak et al., 1993; Thomas et al., 1993; et al., 1991; O Pellizzari et al., 1993; Wallace et al., 1993). Elemental composition of the samples was analyzed by X-ray flu€ zkaynak et al., orescence (XRF) (Clayton et al., 1993; O 1996). The original PEM pumps were made lighter and less noisy by replacing the heavy battery pack with lighter Lithium batteries, simplifying electronics and adding soundproofing material (Spengler, 1994; personal communication). A stratified probability sample of 178 people carried personal monitors for 24 h at the time for two 12-h samples. The particle concentration inside and outside of the home of each participant was measured with stationary PM10 and PM2:5 monitors. Ambient air levels were monitored at fixed sites with high volume PM10 samplers. Daytime personal PM10 exposure levels, as well as the personal exposure levels of nearly all particle bound elements were elevated relative to indoor and outdoor levels. Nighttime personal exposure levels were lower than outdoors but higher than indoor levels. In the Teplice health study in the Czech Republic District of Northern Bohemia, personal and ambient air

Table 2 Summary of selected personal and microenvironmental PM sampling studies, basic design characteristics and sampling equipment used Site

No. of subjects

PM

Smpl. time

Personal sampler

Dockery and Spengler (1981) Sexton et al. (1984)

Watertown MA, Steubenville OH Waterbury, VT

37 volunteers

RPM

12 h

48 volunteers

RPM

2 wk

Spengler et al. (1985) Lioy et al. (1990) Thomas et al. (1993)

Rural TN Phillipsburg, NJ Riverside, CA

Janssen et al. (1997, 1998a,b, 1999)

Amsterdam and Wageningen, The Netherlands Leeds, UKa Stockholm, Sweden etc. Toronto, Canada

RPM PM10 PM10 PM2:5 PM10 PM10 PM2:5 TSP RSP

Ramstrom et al. (1999)

Helsinki, Finlandb Athens, Greece, etc. New York, NY

97 volunteers 14 volunteers 178 randm non-smk > 16 y 37 elderly 45 children 13 children 188–255 nonsmoking in each city 142 randm > 16 922 randm > 16 201 randm adults, 50 in each

Dorr-Oliver cyclone, MSA-G or Bendix BDX pump Bendix cyclone, Harvard/EPRI pump Cyclone, Harvard/EPRI pump MS&T impactor, personal pump MSP impactor (PM10 ), Casella/ BGI AFC 400 pump MS&T impactor, Gillian Gil-Air 5 pumps, Casella respirable dust cyclone SKC Mod 222-3 pump, 0 mm Dorr-Oliver cyclone, SKCAircheck Mod50 pump MSP PEM, battery pump

Lanki et al. (1999)

Sarnat et al. (2000)

Phillips et al. (1994, 1996, 1997a,b,c, 1998a,b) Pellizzari et al. (1999) Jantunen et al. (1998)

a b

14*24 h 2*12 h (4–8)* 24 h 24 h

Flow rate

Microenvironment sampler

Flow rate

Same as personal

4 l/min 4 l/min 4 l/min

139 & 1720 ml/min 2 l/min

Harvard impactor (PM10 & PM2:5 ), MEDO pump Harvard impactor (PM10 & PM2:5 ), ADE SP-280E pump

10 l/min

MSP PEM, battery pump

2.0 l/min

10 l/min

PM2:5

72 h

PM2:5

48 h

BGI GK 2.05-KTL cyclone, Buck IH pump

4 l/min

BGI WINS PM2:5 Impact. BGI PQ-100 pump

16.7 l/min

50 teenagers

PM2:5

48 h

BGI GK 2.05-KTL cyclone

4 l/min

BGI GK 2.05-KTL cyclone

4.0 l/min

Helsinki, Finland

47 elderly

PM2:5

24 h

4 l/min

Baltimore, MD

20 elderly

PM10 PM2:5

2*12* 24 h

BGI GK 2.05-KTL cyclone, MIE pDR-1200 monitor, BGI 400 pump 2 PEMs, one BGI 400 pump

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Study

2 & 3.2 l/min

Leeds (UK), Stockholm (Sweden), Bremen (Germany), Barcelona (Spain), Turin (Italy), Lisbon (Portugal) and Paris (France). Athens (Greece), Basel (Switzerland), Grenoble (France), Helsinki (Finland), Milan (Italy), Prague (Czech Republic).

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M. Jantunen et al. / Chemosphere 49 (2002) 993–1007

exposures to PAH and organic mutagen exposures were monitored using a lightweight integrated sampler. The sampler, consisting of a PM2:5 impactor, secondary impregnated filter for sampling nicotine, and followed up by an XAD resin filled cartridge to sample semi-volatile organic compounds, was worn in the breathing zone (Watts et al., 1994; Williams et al., 1999). In the Netherlands Janssen et al. (1997, 1998a,b, 1999) conducted a panel study in 1994–1995 on personal PM10 and PM2:5
3 exposures and microenvironment concentrations of schoolchildren and adults in Amsterdam and Wageningen (a small University town). For personal PM10 sampling they used the same single hole MS&T impactor samplers with 25 mm 3 lm pore Teflon filters (Gelman R2P1025) that were used in the THEES study. The samplers were attached to clothing near the breathing zone. Air pumps, carried in a bag, were used to draw the samples at 4 l/min. Pump noise was reduced using a plastic container filled with absorption material. In the night at home the assembly was placed into a sound absorbing container for additional noise reduction. Personal fine PM sampling was conducted by respirable dust cyclone developed for industrial hygiene sampling of PM5 at 1.9 l/min. By increasing the sampling flow rate to 4 l/min, it was calculated to have median cut size at 3 lm. Indoor PM10 in this Dutch study was sampled using the Harvard Impactor with a 10 l/min flow controlled AC power driven pump. The indoor PM2:5 was sampled using the PM2:5 version of the Harvard Impactor driven by the same pump. A large European multi-center study to evaluate fine PM exposures and the role of ETS exposure in particular was conducted in Leeds (UK), Stockholm (Sweden), Bremen (Germany), Barcelona (Spain), Turin (Italy), Lisbon (Portugal) and Paris (France) (Phillips et al., 1994, 1996, 1997a,b,c, 1998a,b). In the earliest measurements in Leeds, a personal sampler was used to collect ‘‘particles from all sources’’. Two filters were used, the first Teflon filter (FALP 02500, Millipore UK Ltd., Hertfordshire, England) to collect the particles, and the second (Fiberfilm T60A20, Pallflex Corp., CT, USA) acidified with sodium bisulfate to collect nicotine. A lightweight battery operated pump was used to draw air at 139 ml/min to collect a 200 l sample in 24 h. This setup could not distinguish between the inhalable and noninhalable particles. Due to very small sample size it also had a high detection limit of 20 lg/m3 . Later for the other cities (Stockholm . . . Paris) a higher volume sampler with a respirable particle (PM3:5 ) cyclone was used to remove the coarse PM fraction before sampling on a 37 mm 1-lm pore fluoropore membrane filter (FALP 03700). A XAD4 resin filled collector tube in series with the RPM cyclone was used to collect vapor phase nicotine and 3ethenylpyridine. Both samples were drawn by the same battery operated personal pump that was adjusted for

particle sampling to 1.7 l/min and collected the vapor phase at approximately 0.8 l/min. No microenvironment samples were collected in these studies. A large-scale study to evaluate human exposure to fine PM (PM2:5 ) manganese from the use of gasoline additive, methylcyclopentadienyl manganese tricarbonyl (MMT) in Toronto, also produced data on personal exposures to PM2:5 (Pellizzari et al., 1999). Identical sampling equipment was used for personal sampling and residential indoor and outdoor sampling. The microenvironment sampler was placed in a better-soundproofed, larger and heavier container. The sampling pump used in this study was battery operated (4 AA cells), with automatic on/off control (3 min/1 min) through the 3day monitoring period. A motion detector was used record the periods of subject activity. The setup was remarkably lightweight (slightly less than 1 kg) and was carried in a waist pack. Small inertial 2 l/min impactors were constructed. The PM2:5 impactors were fitted with a 4.5 lm scalping stage to avoid impaction stage overloading. In the large European multi-center, multi-pollutant personal exposure study EXPOLIS, personal, home indoor and outdoor and workplace concentrations of PM2:5 , 30 VOCs, CO and NO2 were measured. Population samples ranging from 50 to 201 subjects were measured in six cities (Athens, Basle, Grenoble, Helsinki, Milan and Prague). Identical equipment was used and standard operating protocols (SOP) were followed in each center. (Jantunen et al., 1998; Koistinen et al., 1999). The new feature in the EXPOLIS design was sample timing. The samples were not collected for prefixed 24 or 12 h periods (like THEES) or separately for day and night hours (like PTEAM), but instead were programmed individually to monitor separately the working hours, including commuting, and the leisure time of each subject. The EXPOLIS PM2:5 exposure monitoring equipment, (sampling pump, 2.5 lm cyclone, 37 mm holders with filters, and a battery pack––together with VOC sampler and CO monitor) was packed into a 4.5 kg (total) aluminum briefcase that each subject carried or kept within arms reach for 48 h. The modified Buck air pump was lightweight and after modification capable of sampling up to 60 h with a single set of batteries. It was adjusted to draw air at 4 l/min using a simple volumetric flow control. The small PM2:5 cyclones for personal PM2:5 sampling at 4 l/min were designed and constructed for the EXPOLIS study by BGI, Inc. With this design the filters were handled from pre- to post-weighing in standard 37 mm plastic filter holders minimizing the risk of filter contamination and damage in the field. Two filter holders with 2-lm pore Gelman Teflo (Gelman Sciences, Ann Arbor, MI) filters were provided for each subject: one for the two workday sampling periods and the other for the remaining times.

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The same personal PM2:5 cyclones were also used in the EXPOLIS study in Oxford (Kendall et al., 1999) and London, and the Harvard-Columbia conducted Leland study of schoolchildrenÕs exposures in New York City (Ramstrom et al., 1999). The EXPOLIS workplace and home indoor and outdoor microenvironment monitors (MEM) were programmed to run inside and outside of the home for the expected non-working hours and in the workplace for the expected working hours of each subject. The MEM sampler used a WINS PM2:5 impactor (The EPA designated Federal Reference Method for PM2:5 sampling), a 47 mm filter holder with a 2-lm pore Gelman Teflo filter and a PQ100 pump (BGI, Inc.). The PQ100 pump is weatherproof, equipped with a microprocessor-controlled timing and mass flow adjustment system, and capable of operating up to 36 h on a fully charged internal lead-acid battery. Sarnat et al. (2000) studied the exposures of 20 elderly non-smoking individuals living in Baltimore, MD, to PM10 , PM2:5 , SO2
4 , O3 , NO2 , SO2 and selected VOCs. The 24-h measurements were repeated for each individual 12 times in the summer and in the winter of 1998–99. Only personal exposures were measured. The innovative feature of this PM exposure study was that two six-hole impactor samplers (similar as in PTEAM) were used, one to sample PM10 at 2 l/min and other to sample PM2:5 at 3.2 l/min on 37 mm Gelman Teflo filters. The samplers were fitted, together with the passive O3 , NO2 and SO2 tubes into a pack of two aluminum elutriators (vertical laminar flow tubes to remove large particles by sedimentation) and carried in the breathing zone of each participant. One sampling pump was used to collect both PM samples and the 20 ml/min VOC sample via split tubing and flow restrictors.

1.4.2. Developing world Most of the personal and indoor exposures to combustion generated PM in developing countries are due to the burning of biomass fuels in the form of wood, agricultural waste and animal dung. The smoke from these fuels, containing hazardous suspended PM including carcinogenic polycyclic aromatic hydrocarbons, affect mainly women who do the cooking in the rural villages. It has been estimated that 300–400 million people are affected worldwide, mostly in the rural areas of developing countries (De Koning et al., 1985). Mumford et al. (1987) studied the PM and PMbound carcinogenic exposures and respective lung cancer risks of women cooking with coal and wood in Xuan Wei, China. They used standard HiVol samplers with pre-separators to sample PM10 and medium-volume PM10 samplers with XAD-2 resin traps to collect semivolatile organic compounds. Personal 1 l/min PM samplers were used to collect samples for scanning electron

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microscope (SEM) characterization and elemental analysis by XRF. Cooking can result in highly elevated indoor PM pollution (Sofoluwe, 1968; Gujarat, India, Smith et al., 1983; Zimbabwe, Collings et al., 1990) and more specifically PM bound PAH pollution (Kenya, Clifford, 1972; India, Smith et al., 1983), even when the outdoor concentration may be low. In more advanced developing countries like Korea (Lee et al., 1997), Taiwan (Li, 1994) and Malaysia (Zailina et al., 1996), indoor air quality reflects more the conditions in developed nations. These countries are undergoing rapid urbanization and industrialization, and indoor air quality is strongly influenced by motor vehicles and industrial sources outdoors, as well as smoking and gas cooking indoors. Table 3 summarizes studies on indoor air pollution from biomass combustion in developing countries (Smith, 1986), as well as other later studies on general indoor air pollution. The table shows that the sampling methods used in the recent studies in the developing world are essentially the same as the ones used in North America and Europe. In the reports of some early studies the sampling and analysis methods, sampling times etc. are not given. 1.4.3. Other PM-metrics: BS, BC and particle numbers Reponen et al. (1996) compared the measurements of black smoke (BS), and black carbon (BC) measured with an aethalometer (Magee Scientific) with size fractionated particle count data measured by Electrical Aerosol Spectrometer (Tartu University, Estonia. 12 size fractions between 0.01 and 10 lm). Both BS and BC data correlate very well with the PM1:0 particle numbers (r2 ¼ 0:91 and 0.93 respectively). As the BS and BC analyses are not gravimetric but optical, they require only small sample volumes, and can be considered as indirect, but in many cases quite useful, proxies for the PM1:0 particle numbers. BS measurement has the additional advantage that it is quite cheap and easy to perform by a standardized method (OECD, 1964; ISO, 1993). Analysis equipment is available and abundance of reference data has been collected worldwide over the last decades. BS is also a relatively good proxy for the more expensive and demanding BC. Both BS and BC can be considered to be linked with combustion-generated primary PM. Increasing emphasis on combustion-generated PM and respective de-emphasis on soil and vegetation PM may renew the interest of epidemiologists to BS data and methods. As an interesting curiosity and a low cost alternative to ambient or microenvironment air pollution monitoring an Australian program about air quality issues has developed, in partnership with regulatory agencies and CSIRO (Commonwealth Scientific and Industrial Research Organization, Australia) an AirWatch sampler.

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Table 3 Indoor air pollution studies in developing countries. Basic design, PM results and equipment Site

No. of subjects

PM sampled

Sampling time

Cleary and Blackburn (1968) Smith et al. (1983)

Papua New Guinea

9

TSP

All night

Gujarat, India

70 women

TSP

45 min

Mumford et al. (1987) Boleij et al. (1989)

Xuan Wei, China Rural Kenya

12 homes

PM10

550–60 min 4 h cooking session

134

TSP PM10

24 h

Collings et al. (1990) Raiyani et al. (1993) Li (1994)

Zimbabwe

40 homes

TSP

2h

India

20

TSP

10 cooking sessions

Taiwan

365 838

24 h

None

–

Rural Mexico

6664

9h

None

–

Chongju, Korea

16

TSP PM10 PM2:5 PM10 PM2:5 PM2:5

24 h

None

–

Brauer et al. (1996) Lee et al. (1997)

Personal sampler

Flow rate

Microenvironment sampler

Flow rate

MSA pump Casella sampler Model 3110/TT Not named

Not given

Not given

1 l/min

None

–

MSA pump & Casella sampler, mod 3110/TT HiVol with PM10 SSI MedVol PM10 & XAD DuPont P2500 pump & PAS-6 filter holder Casella cyclone Casella sampler, mod 3131/TT Andersen cascade sampler SKC pump & MSP mod 200 PM10 & PM2:5 sampler URG-200Q pump Inertial impactor Annular denuder system & cyclone

1200 l/min 110 l/min 2 l/min

1.7 l/min 28.3 l/min 3.5 l/min 10 l/min 10 l/min 4 l/min 10 l/min

M. Jantunen et al. / Chemosphere 49 (2002) 993–1007

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The sampler consists of a plastic filter holder (followed by another filter holder for NO2 absorption), a dry gas meter, an aquarium aerator pump and a 12 V lead-acid battery. The gray shade of the filter is visually compared to a grey scale standard. The method is validated and produces BS compatible results. By in situ calibration it can also be used for estimation of PM10 levels (Manins et al., 1998). Recently there has been renewed interest, supported by some epidemiological and toxicological evidence on the health effects of particle numbers, especially the numbers of the s.c. ultrafine particles instead of PM mass. The ultrafine particle numbers are not correlated with PM2:5 mass, and cannot therefore logically neither explain nor contradict the observed associations between fine PM mass and health effects. There are, however, definitely new interests also on fine particle numbers and possibly other fine PM metrics.

2. Present personal and microenvironment PM10 and PM2:5 samplers Present personal PM10 and PM2:5 samplers use either sharp cut impactors or cyclones to separate the particles with aerodynamic diameters larger than 10 or 2.5 lm. The pumps used in these samplers can operate up to 60 h (Jantunen et al., 1998; Pellizzari et al., 1999) with a single set of batteries. The focus of personal PM sampling has moved from RSP to PM10 and recently PM2:5 . The commercially available personal fine PM samplers are either designed for RPM or personal PM2:5 or PM10 as defined by the USEPA FRM compatibility requirements. RPM samplers are not discussed in this paper. Personal PM2:5 samplers for sampling times from 12 h upwards and with sampling rates 2–4 l/min are based on impaction (e.g. MSP PEM Model 200, SKC PEM Mod 761-203A) or centrifugal separation (e.g. BGI cyclone, GK 2.05KTL), or a combination of both principles (e.g. SKC Spiral sampler, Mod 225-201). The main advantages of impactors are relative insensitivity to flow rate fluctuations, small size and light weight. The main disadvantages are the particle bounce risk and usually cumbersome filter changing. The main advantage of the cyclone sampler is the fast and easy filter handling; the filter is packed in a plastic filter holder and untouched from pre- to post-weighing. On the other hand the cyclone cut size is more sensitive to flow rate fluctuations, and the cyclones are larger and heavier. Some new continuously recording personal PM monitors are sufficiently sensitive for urban environmental monitoring, battery powered, relatively lightweight, and have data logging capacity for hundreds or thousands of measurements. An example, the Personal DataRAM monitor (MIE Bedford, MA), is based on

1001

passive airflow and forward light scattering principle. It is most sensitive for 0.1–10 lm particles at concentrations down to 1 lg/m3 . Optical aerosol monitors, however, do not measure PM mass. They are sensitive to particle size distribution, aerosol optical characteristics and relative humidity of the air, and need therefore be calibrated for each measuring condition and aerosol type separately. This limits their applicability in personal exposure monitoring, where both the aerosol characteristics and the ambient air conditions may change while the person moves from one microenvironment and activity to another. The new pDR-1200 combines PM2:5 selectivity, continuous optical aerosol monitoring and collection of an integrated PM2:5 sample on a filter for gravimetric, elemental and/or chemical analyses. It consists of a BGI GK 2.05-KTL cyclone, MIE pDR monitor, 37 mm filter holder and BGI 400 pump in series. The sampler was first applied in the EU funded ULTRA study for real time monitoring of personal PM2:5 exposures of cardiovascular outpatients in Helsinki (Lanki et al., 1999). SKC produces a similar continuous PM10 , PM2:5 or PM1:0 monitor/sampler (SKC EPAM-5000). Both the pDR-1200 and SKC EAPM-5000 are packed as an independent, self contained unit for carrying in the field. Practically the only PM sampler originally designed for non-occupational microenvironment monitoring is the s.c. Harvard Impactor, commercially available at least from one manufacturer (Air Diagnostics and Engineering, MS&T Area Sampler, and MSP MEM Model 400). The Harvard impactor is available with cut sizes for PM10 , PM2:5 and PM1:0 with flow rates at 4–20 l/min (Turner et al., 2000). Because the standard ambient air PM10 and PM2:5 impactors and cyclones are operated at low flow rates (e.g. 16.7 l/min) they are fully applicable for indoor and outdoor microenvironment sampling as well. Because they need to be proven EPA Federal Reference Method equivalent, they are usually backed with quite good performance documentation (e.g. BGI PQ200, Ruprecht and Patashnick Partisol, Andersen Graseby RAAS2.5, etc.). The main challenge in using ambient samplers in indoor microenvironments is sufficient soundproofing of the air pump. Continuous monitoring of ambient air PM10 or PM2:5 mass has proven more difficult than expected. The most common measurement principles are TEOM (Tapered Element Oscillating Microbalance, Rupprecht and Patashnick Co., Inc.) and beta radiation attenuation (e.g. BAM 1020, Met One Instruments). Both principles measure truly mass, but cannot distinguish between water and other PM mass. If uncontrolled, this phenomenon leads to highly biased concentration results in changing temperature and/or relative humidity conditions. The problem has been attempted to overcome by heating the PM sample to e.g. 50 C, but this creates a

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Table 4 Contact information for manufacturers of personal and (micro)environmental particle samplers and monitors (alphabetial order) Manufacturer

Address

http://www.-location

Air Diagnostics and Engineering, Inc. Airmetrics Andersen Instruments, Inc. BGI, Incoporated Casella Group, Ltd. Gillian Instruments, Corp. GreenTek, Inc. Met One Instruments, Inc. MIE, Incorporated MSP Corporation Particle Measuring Systems, Inc. Rupprecht & Patashnick Co., Inc. SKC, Incorporated TSI, Incorporated

Harrison, ME 04040, USA Eugene, OR 97403, USA Smyrna, GA 30082, USA Waltham, MA 02451, USA Kempston, Bedford, UK West Caldwell, NJ, USA Grants Pass, OR 97526, USA Grants Pass, OR 97526, USA Bedford, MA 01730, USA Minneapolis, MN 55414, USA Boulder, CO 80301, USA Albany, NY 12203, USA Eighty Four, PA, USA St. Paul, MN 55164-0394, USA

AirDiagnostics.com Airmetrics.com AndersenInstruments.com BGIUSA.com Casella.co.uk GreenTekUSA.com MetOne.com MIEInc.com MSPCorp.com PMeasuring.com RPCo.com SKCInc.com TSI.com

The list is not complete because of continuous development and difficulty in finding some manufacturerÕs home pages.

new problem; the nitrates or other volatile PM constituents evaporate from the sample during the heating. In indoor microenvironments, where the temperature and relative humidity remain relatively stable, both TEOM and Beta Attenuation are applicable for continuous PM monitoring. One new and interesting technique for continuous PM2:5 monitoring is the Harvard-developed CAMM (Andersen Instruments). CAMM is based on measuring the pressure drop across a porous membrane filter (Koutrakis, 1997). This instrument does not require sample heating. The pressure drop is proportional to the accumulated mass, but sensitive to changes in air humidity, temperature and flow. The instrument monitors the difference between two pressure drops, one through a loaded and the other through an unloaded filter to balance these effects. Certainly, these are only examples of the most used and most recent personal and microenvironment PM samplers. Other manufacturers, techniques and models exist and new developments take place constantly. Table 4 presents an unavoidably incomplete list of PM sampler manufacturers with contact information valid at the time of submission of this manuscript. The most striking feature of this list is that almost all the identified manufacturers are American. 3. Development needs Over ten years ago NRC set six criteria for new personal monitors (NRC, 1991). The desired characteristics, modified and abbreviated here for personal PM monitoring would be the following: • Sensitive to changes that are 1=10 of the level of interest, have precision of 5% and be easy to calibrate accurately.

• Selectivity (e.g. PM2:5 size cut). • Rapidity: sampling and analysis times should be short compared with biological response times. • Portability: the device should be rugged and not interfere with the normal behavior of the subjects; battery operated. • Cost: sampling and analysis should not be prohibitively expensive. Since 1991 the possibilities and requirements have increased. Lioy (1993) stated that one main requirement of future research is that the investigators developing equipment must recognize that the devices are to be worn by people and that the monitor must be tested to ensure that the participants in an exposure study can wear it comfortably. For personal PM samplers, this requirement is still rarely met.

3.1. Monitoring equipment NRC Research Topic 3 (NRC, 1998) defines requirements for the next generation of personal PM samplers. The following discussion elaborates over a longer development period. The requirements for an ideal personal PM sampler include continuous concentration (or particle count) recording and collection of full particle size distribution data. It senses and records its environment to allow direct linkage of exposures and microenvironments. It is programmable for scheduled operation, it controls its flow rate at the set level and records any irregularities in the sampling process. It runs for a week independently without operator service. So far we are quite far from most of these ideals. Based on the presently available techniques, the first development needs are:

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• Reducing the sampler size, weight and noise level. This could be made possible by significantly reducing the necessary sample size and flow rate. This elevates the requirements for the filters, and for the weighing and analytical laboratory. • Producing a (semi) continuous record of the particle mass/number concentration and/or particle size distribution instead of one integrated sample. • Microenvironment sensing could be based on pattern recognition of data from a set of standard sensors; e.g. temperature, noise, vibration, light, etc.

of the brand and often even the batch needs to be tested for a particular use. Filter pre-processing, like thermal treatment or washing may provide a solution for some contaminants. An ideal PM filter would not create static electrical charge; it would have high collection efficiency, low flow resistance and low tare mass. It would be physically robust, chemically inert and stable, and have negligible adsorption/desorption capacity for water and other volatile compounds.

An ideal microenvironment sampler would have most of the same properties as the personal exposure sampler. It could be larger and heavier but its silence is as important as with the personal sampling. A microenvironment sampler does not need to detect its microenvironment, but detecting the activities around it would be an advantage.

3.3. Laboratory methods The filter conditioning and weighing procedures in the laboratory are equally important in the determination of PM concentrations. As we want to use lighter and smaller samplers and lower particle size cut points, the relative importance of high quality laboratory procedures increases.

3.2. Filter media Different types of filters are needed for different sampling purposes. Glass fiber filters are cheap, but their fragility and hygroscopicity necessitate relatively large samples for reliable net mass determination. Most glass fiber filters contain both inorganic (glass) and organic (resin) contamination and are therefore only conditionally applicable for elemental or chemical analyses of the samples. Similar in hygroscopicity, appearance and fragility are the more expensive quartz fiber filters, which, due to high thermal resistance and the absence of any carbonaceous material in the filter, are particularly applicable for elemental carbon (EC) and organic carbon (OC) analyses from the samples. Membrane filters, such as mixed cellulose ester (MCE) have high flow resistance, collect the particles on the filter surface which is ideal for PIXE analyses, are fully soluble for wet chemical analyses, and have low hygroscopicity, own mass, inorganic and organic contamination. PVC has good filtering characteristics and is applicable for e.g. silica free sampling needs. The presently popular Teflon filters are rather expensive, but also non-hygroscopic and chemically inert. Like other organic filter materials, they cannot be used for sampling for EC and OC analyses, they are problematic in PIXE analyses (due to poor penetration of the protons into the depth of the filter matrix), but quite suitable for XRF analysis. Blank Teflon filters absorb material from the surrounding air even when not used for sampling (Thomas et al., 1993; Janssen et al., 1998a; Koistinen et al., 1999), and on the other hand loose mass when warmed to 40–60 C. The 2 lm pore Teflon filters get easily overloaded even in moderate ETS exposures. All filters are subject to contamination during the manufacturing process, and therefore the applicability

3.3.1. Flow calibration The primary standard for flow rate measurement is the soap bubble flow meter. TodayÕs semiautomatic soap bubble flow meters are small and practical. When calibrating a sampler with a built in mass flow controller, it is important to realize, that while a mass flow controller adjusts the mass flow rate, the bubble flow meter measures the volume flow rate. The actual mass flow rate depends then on the air density, which is a function of air pressure, temperature and relative humidity. Temperature and humidity can be controlled in a laboratory, but air pressure not. The natural variation of atmospheric pressure can change air density up to 10% in a matter of a day. 3.3.2. Filter weighing Most PM concentration values are calculated from the small net differences (lgÕs) of two large readings (mgÕs); the pre- (tare) and post-sampling (gross) filter weighing results. Except the sampled PM, this observed difference also depends on: (i) forces other than gravity affecting on the filter as it is being weighed, • electric charge • air buoyancy • centrifugal force due to earth rotation • vibrations (ii) conditions that affect the balance stability (iii) changes in the mass of the filter • water content due to variation in relative humidity • volatile material (e.g. organics, nitrates) • (de)contamination

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These phenomena were statistically analyzed by H€ anninen et al. (2002) from the approximately 1000 weight measurements of 23 laboratory blank filters in the EXPOLIS study, Helsinki. Errors caused by static charge, when present, were found to be two orders of magnitude higher than the others are (Koistinen et al., 1999; Lawless and Rodes, 1999; H€anninen et al., 2002). Static charge can be removed by commercial static charge removers based on either ionizing radiation from a strong alpha source (typically 500 lCi of Po-210, halflife 138 days) or corona discharge (e.g. Multistat). Meteorological variation in air pressure is the main cause for variation in air density, which changes the buoyancy force, which makes the filter appear lighter. When the air pressure and thus the air density changed between two consecutive weight measurements, an error up to 10 lg was introduced (H€anninen et al., 2002). This buoyancy effect can be eliminated, if the air pressure, temperature and humidity in the weighing room for each weighing session are recorded, by applying ideal gas law (Allen et al., 1999; Koistinen et al., 1999; H€ anninen et al., 2002). The balance stability (ii) needs to be monitored by repeated weighing of a chemically inert and physically stable calibration standards. The standard weights should have approximately the same mass as the weighed filters. The impact of filter conditioning/weighing room air relative humidity level has two components. The first is the impact on the filter, which can be minimized by using non-hygroscopic filters (e.g. Teflon) and numerically removing the remaining filter hygroscopicity effect by an experimental filter mass/RH regression model. The second component is the impact of the RH on hygroscopic components of the sample. The main PM2:5 components that are both abundant and hygroscopic are salts and sulfates. Both will be water free when the sampled filters are first desiccated in dry air (to remove the effects of water adsorption–desorption hysteresis) and then conditioned in a weighing room, where air RH is kept below 50%. The impact of filter handling on the filter mass may have a high uncorrectable random component. It can be reduced by laboratory and field practices that minimize filter handling, e.g. by keeping the filters untouched in sampling cartridges from pre- to post-sampling weighing. The errors caused by absorption of mass on the filter and filter material volatilization can be minimized by keeping the time between weighing as short as possible. Volatilization can also be controlled by keeping the filter cool, and sometimes by thermal pre-treatment. The rather large observed blank filter mass increases (Thomas et al., 1993; Janssen et al., 1998a; Koistinen et al., 1999; H€ anninen et al., 2002) indicate that the absorption is location (and possibly time) specific. The compounds causing this mass increase have not been identified. So

far the only available method for reducing this error would be by removing its mean daily impact based on blank filter data from the same location, time and filter type as in the actual sampling. 3.3.3. Climate control Key issues in the climatic control or monitoring of the PM sampling in the field, flow rate adjustment and measurement in the field and filter weighing in the laboratory are air density (determined by pressure and temperature), and humidity. In principle the errors caused by changes in these variables can be controlled either by keeping them constant, or by monitoring them and correcting the results numerically. In the field the latter way is the only alternative. In the laboratory the temperature and humidity can be controlled using thermostatic heating and air conditioning. Natural day to day changes in atmospheric pressure cannot be controlled. Changes in temperature, humidity and pressure, however, are all easy and cheap to monitor with considerable accuracy. Therefore, simple air conditioning and dehumidifying combined with the use of non-hygroscopic and thermally stable filters and numerical control of the impacts of residual climatic variability on flow measurements and filter weighing in the laboratory may provide an economical alternative to climate control.

4. Conclusions The progress is leading from 4 l/min towards lower personal sample flows, lighter, silent, independent battery powered ÔsmartÕ monitors with data logging capacity to store microenvironment and subject-activity relevant sensor data, advanced flow controls and continuous recording of the concentration. The best PM filters are non-hygroscopic, chemically pure and inert, thermally stable, and physically robust, and they are kept protected in filter holders from pre- to post-sampling weighing. Semiautomatic and primary standard equivalent positive displacement flow meters are replacing less accurate methods in flow calibration, and the pump flow rates in not only microenvironment but also personal samplers are becoming mass flow controlled, with or without volumetric compensation for pressure and temperature changes. In the weighing laboratory the effects of air pressure variation are controlled in addition to the effects of humidity and temperature. Repeatability measured as standard deviation of duplicate samples, of 1.5 lg is now achievable. In a 12-h, 2 l/min sample this corresponds to 1 lg/m3 . The advantages from highly accurate and precise sampling and laboratory equipment/procedures are not only better data, but also the possibility to shift to lighter, smaller, cheaper and less noisy sampling, or shorter sampling

M. Jantunen et al. / Chemosphere 49 (2002) 993–1007

times. These factors allow for less invasive and more representative studies. Acknowledgements This work was initiated, made possible and managed by the WHO/SCOPE/SGOMSEC secretariat, and by the EC: JRC Environment Institute/Air Quality Unit that has strongly supported the work of M. Jantunen in the past year. References Allen, G., Oh, J.A., Koutrakis, P., Sioutas, C.J., 1999. Techniques for high-quality ambient coarse particle mass measurements. Journal of Air and Waste Management Association 49, 133–141. Anderson, R., Kamens, R., Rodes, C., Wiener, R., 1989. A collocation study of PM10 and PM2:5 inertial impactors for indoor aerosol exposure assessment. Proceedings of the 1989 EPA/APCA International Symposium: Measurement of Toxic & Related Air Pollutants. Boleij, J.S.M., Ruigewaard, P., Hoek, F., Thairu, H., Wafula, E., Onyango, F., DeKoning, H., 1989. Domestic air pollution from biomass burning in Kenya. Atmospheric Environment 23, 1677–1681. Brauer, M., Bartlett, K., Regalado-Pineda, J., Perez-Padilla, R., 1996. Assessment of particulate concentrations from domestic biomass combustion in rural Mexico. Environmental Science and Technology 30, 104–109. Buckley, T.J., Waldman, J.M., Freeman, N.C.G., Lioy, P.J., 1991. Calibration, intersampler comparison, and field application of a new PM-10 personal air-sampling impactor. Aerosol Science and Technology 14, 380–387. CASAC, 1997. (Clean Air Scientific Committee) of EPAÕs Science Advisory Board (1997) Evaluation of Research Needs for the Particulate Matter National Ambient Air Quality Standards (NAAQS), Letter to Administrator Browner from the Clean Air Scientific Advisory Committee, EPA-SAB-CASAC-LTR-97-004, March 12, 1997. Chow, J., 1995. Measurement methods to determine compliance with ambient air quality standards for suspended particles. Journal of Air and Waste Management Association 45, 320–382. Clayton, C.A., Pellizzari, E.D., Wiener, R.W., 1991. Use of a pilot study for designing a large-scale probability study of personal exposure to aerosols. Journal of Exposure Analysis and Environmental Epidemiology 1, 407–421. Clayton, A., Perrit, R.L., Pellizari, E.D., Thomas, K.W., Whitmore, R.W., Wallace, L.A., Ozkaynak, H., Spengler, J.D., 1993. Particle total exposure assessment methodology (PTEAM) 1990 study: Distributions of aerosol and elemental concentrations in personal, indoor, and outdoor air samples in a southern California community. Journal of Exposure Analysis and Environmental Epidemiology 3, 227–250. Cleary, G., Blackburn, C.R., 1968. Air pollution in native huts in the highlands of New Guinea. Archives of Environmental Health 17, 785–794.

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