A method to select locations for indoor air quality sampling

BuddmgandEnmronment, Vol 18, No 4, pp 171-180, 1983

0360-1323/83 $3 0 0 + 0 0 0 © 1983 Pergamon Press Ltd

Printed m Great Britain

A Method to Select Locations for Indoor Air Quality Sampling E A B MALDONADO* J E WOODS1" A rational method is presented to determine the locations wzthm a buddmo where the Mohest averaoe concentrations of contaminants may occur Usm9 this procedure, the number of samphng points necessaryfor mdoor air quahty(1AQ)evaluationof abuddmgisreduced toammimum Thusthetzme and cost necessary for building evaluation and analysis can be decreased Experimental measurements made m a research house are presented to vahdate the method

NOMENCLATURE

the amount of air exchanged Thus, specified amounts of ventilation air have resulted from a compromise between acceptable mdoor air quality and economics or energy availabihty Until the nineteenth century, lack of scientific knowledge left the amount of outdoor mr introduced into buildings,mostly by infiltrationand natural ventilation,to empirical practices In 1824, Tredgold [1] proposed the first quantitattve attempt to control mdoor air quahty, ratlonahmng the need to supply 2 l/s (4 cfm) of outside air per person Over the years, as shown in Fig. 1, the recommended amount of ventilation mr increased to a high of 15 l/s, although current recommendations of 2 51/s are once again sirmlar to those proposed by Tredgold 1"2]. These currently recommended ventilation rates are an mdirect consequence of the continuous escalation of fuel prices which started in the early 1970s Specific ventilatmg rates, such as those in Fig. 1, have usually been applied only to buildings with mechanical ventilation systems In all other buildings, residences in particular, infiltration and natural ventilation have been relied upon for their air exchange needs. In many of these buildings energy conservation measures have been taken which included tightening of the building boundary wtthout considenng ventilation requirements. Lower air exchange rates thus resulted which led to higher contaminant concentrations indoors. In many cases, the levels that were reached caused serious physical problems to the occupants and focused attention upon the problem of indoor air pollution [4, 5] As a result, research programs were carrted out which extenstvely surveyed mdoor air quahty in buildings ['6, 7]. In these programs, each building was surveyed m several locations for long periods of time and for a large number of contaminants. Such an expensive survey can only be justified in research programs and thus cannot be used for generalized surveys of normal buildings For a generalized survey method to be feasible, the duration of the procedure and the number of sampling points must be hmited to a minimum In the following sections, an objective criterion to select the location and number of sampling points will be presented and discussed The discussion wdl be limited to

C instantaneous concentration of a contaminant (e g ~ug/ma) (~ average concentration of a contaminant for a specified period of exposure ttme At (e g #g/m3) E relative exposure index N number of zones m a building V volume of an enclosed space (e g m 3) I;" volumetnc air exchange rate (e g m3/h) net generation rate of a contaminant 0 e source strength less sink strength) (e g #g/h) t Ume at which an mstantaneous concentratton C occurs (eg h) At

exposure U m e that an occupant recurs (e g yr)

Subscripts

a t j n r

outdoor air zone to be evaluated all zones except 1 effectivezonal condmon referencezone INTRODUCTION

TO ASSESS the health and comfort aspects of indoor environments, the quahty of the air inside a building must be evaluated on the basts of(I) the adequacy of its thermal properties (i e dry-bulb temperature, relaUve humidity, and velocity) to provide thermal acceptability for the occupants, (2) the suitabihty of its oxygen and carbon dioxide concentrations to allow normal breathing, and (3) the absence of gases, vapors, and aerosols m concentrations which may have deleterious effects or that can be perceived as objectionable by the occupants The most widely used method of producing acceptable indoor air quality involves introducing outdoor air into the budding for oxygen supply and contaminant dlluhon and condmonlng the air (heating or cooling) to achieve appropriate thermal properUes While large exchange rates of outdoor air might be desirable to keep the concentrations of gases and other pollutants within safe levels, the costs involved m condmonlng this air rise with

* Department of Mechamcal Engmeenng, Iowa State Unlverstty and Umverslty of Porto, Portugal t Departments of Mechanmal Engmeenng and Architecture, Iowa State Umverslty, Ames, IA 50011, U S A 171

E. A B. Maldonado and J. E. Woods

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contaminant concentration measurements, as measurements of the thermal properties of the mr are well known and can be made routinely and inexpensively Moreover, the discussion focuses on residences, because this is the indoor environment where people usually spend most of their time

CRITERION FOR SAMPLING LOCATIONS When a building IS studied for indoor air quality, the objective is to determine whether the occupants may be subjected to contaminant levels which might have deleterious effects Thus, if the highest concentrations of contaminants within the building are located and measured, the locations of the highest risks of exposure for the occupants can be determined The highest concentrations should be interpreted as integrated averages rather than instantaneous values because what most concerns the occupants is the exposure to which they are subjected

±i.

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(1)

In the following sections, the mechanisms that influence contaminant distribution throughout a building and modeling techniques that can be used to characterize them will be examined with the goal of locating where the highest concentration of a contaminant occurs in a building

MECHANISMS FOR CONTAMINANT DISTRIBUTION The concentrations of contaminants within a building may be different from location to location Three main factors Influence the relative magnitude of the concentrations within the building the location and strength of the source, the internal air movements within the different parts of the building, and the type and location of the exchange with outdoor air The nonuniformities throughout the building tend to Increase when the sources and controls (1 e the means of air exchange with outdoors)

are locahzed rather than distributed and when there ISonly limited air movement Internal air movement is of particular Importance to the distribution of contaminants within a building If there were no air movement, the only mechanism for dispersion of a contaminant would be molecular diffusion Such a slow process would result in large gradients of contaminants Conversely, with intense air movement in a building, as in the case of buildings with forced central air distribution, the air tends to be uniformly mixed and equal concentrations may result at all locations For the same generation rate in a building, more localized concentrations occur when less internal air movement exists A similar effect can result from the exchange between indoor and outdoor air The structure of the building boundary and directional effects of the wind, the prime forcing function for infiltration and natural ventilation, can cause different spaces in a building to exchange air with the outdoors at different rates Thus, nonuniform concentrations can also result Although the concentrations may vary from point to point when only naturally induced air movement exists within a building, the magnitude of the variations may, in certain cases, be considered insignificant when compared to the values of the concentrations themselves For example, the differences may be very small within individual rooms, where natural air currents may tend to cause good mixing because of the openness of-the space Conversely, as there usually are only limited openings between rooms, significantlydifferent concentration levels may exist from room to room This effect has been confirmed by earlier reported measurements evaluations of mixing uniformity m rooms, performed by tracer gas methods, showed that good mixing existed in most cases, in particular in rooms of regular rectangular shape [8, 9], but that different rates of decay of tracer gas concentrations were observed in different rooms in buildings [10, 11] To illustrate these differences better, we have performed tests to evaluate the uniformity of mixing in a building (See the Appendix for description of facihty and equipment ) In these tests, a tracer gas (SF6) was uniformly mixed through

A Method to Select Locations for Indoor Air Quality Sampling

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s h o w n in Fig 2 Table 1 summarizes the results that were o b t m n e d and compares them to a test, s h o w n m F~g 3, in which the central a~r-handling system was not deactivated and 'umformly mixed' SF 6 c o n c e n t r a t m n s were amficmlly maintained t h r o u g h o u t the house during the decay process

the m r - h a n d h n g system to obtain the same SF 6 concentrations at all 12 locatmns within the building where the concentrations were concurrently m o m t o r e d Then the mr h a n d h n g system was deactivated and the decay of the SF 6 c o n c c n t r a t m n at those 12 locations was m o n i t o r e d for at least 6 h A typical decay distribution is

Table 1 Variation of concentrations (ppm) ofSF 6 throughout the Energy Research House during tests to evaluate unfforrmty of mixing* Number of probest" Outdoor temperature (°C) Wind speed and d~rect~on (m/s) Indoor temperature (°C) Living room Living room Greenhouse Stmrwell Basement Whole house Fan deactivated during decay Fan acuvated during decay (Fig 3)

Test No 15

Test No 2§

41

-04

58 (S)

66

04 (S)

16

Test No 3

19

38 (W) 20

Test No 4 -48

17

58 (E) 16

0 59-0 63 (3%) -0 21-0 54 (44%) ---

1 99-2 07 (2%) -1 58-2 22 (17%) ---

-3 10-3 14 (1%) 3 05-3 83 (11%) 2 84-3 27 (7%) 286-292 ( 1 % )

-2 75-2 80 2 47-2 95 2 64-2 87 256-257

12

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* Values hsted are minimum and maximum measured m the space at the end of the momtonng penods Values m parentheses are the relatwe magnitudes of the dexaatlons from the rmdpomts of the ranges compared to the values of the mtdpomts oftbe ranges Central mr fan was deacUvated at the start of decay periods t See Fig 10 for probe locations $ Stx probes m the hwng room were placed m the geometrical centers of the east and west halves of the floor plan and wlthm 10 cm of the four comers of the room All were 16 m above the floor § Six probes m the hwng room were placed m the geometrical centers oftbe east and west halves of the floor plan at three different levels 5 crn above floor level, 16 m above floor level, and 5 cm below cedmg level

174

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Although the magnitudes of the rates of decay of SF 6 varied from test to test because of different wind and outdoor temperature condittons, all tests resulted m sirmlar patterns The results listed m Table 1 show that normal rectangular shaped rooms (i e the living r o o m and the basement) indeed had only small variations in concentrations among different points (~<3%) Conversely, rooms with irregular shapes (e g the stairwell) and high ceilings (e g the greenhouse) conststently showed larger vanations (as much as 44%), with higher concentrations at the upper floor than at the lower floor (l e stratification was found in all rooms that spanned more than one level in the building, see Fig 2) Thus, we conclude that individual rectangular rooms with normal ceiling heights (about 2 5 m or 8 ft) can be treated as uniformly mixed for practical purposes when compared to room-to-room and whole-house differences

the ERH with central mr fan energazed

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Diffusion effects between zones are considered neghglble compared to convective effects and thus are not included m equation (2) An equation of this type can be written for each of the N zones m the building, resulting m a system of simultaneous first-order differential equations To solve for the concentrations, however, both the net zonal generation rates q, and the lnterzonal air flows V,j must be known No

ZONE I V1

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MODELING OF CONTAMINANT CONCENTRATIONS IN A BUILDING Exact solutions of the general equations that describe contaminant movement in a space, i e continuity, momentum, energy, and diffusion equahons, are possible only for extremely simple sets of boundary conditions which seldom represent real situations To model actual situations, slmphfymg assumptions are necessary, the most c o m m o n of which is the assumption of perfect uniform mixing m certain zones In this way, a budding can be modeled as a group of N uniformly mixed zones interacting among one another and with the outdoors For a typical zone 0), as represented in Fig 4, the variation of

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A Method to Select Locations for Indoor Air Quality Samplin9 practical, rehable method of measuring these low velocity, turbulent and unsteady flows ~s known and, thus, a system of equatmns of the type of equatmn (2) cannot be used for practmal models of contaminant concentrations m a budding Indirect methods must then be used To do that, equatmn (2) can be rewritten as

dC, = K,C.-P,.C,+ao, '

If the tracer gas is released over a short period oft~me at a particular location m the budding, then the concentrations that result In d~fferent zones of the bmldmg are a consequence of the internal air movement Thus, the exposure m each uniformly mixed zone resulting from the tracer-gas release m a particular location ~s an indication of the relative risk for the occupants at each location and can be obtained from the following equation

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where, for example, the reference zone (r) can be taken as the zone where the tracer-gas release takes place The ratm E has been described as the 'relatwe exposure index' [12] Examples of experimental tests that allow calculatmn of the relatwe exposure radices are shown m F~gs 5-8 for four different locaUons of tracer gas release (same amount of tracer gas released m all four tests) The patterns of concentrations that resulted m all cases were mmtlar

(4)

The t~., term thus accounts for the net generaUon rate of contaminant and for the contaminant removed or introduced into the zone by air exchange with the other zones m the building If the effectwe generation rates can be evaluated, then the concentrations in the N zones can be modeled But the magmtudes of the flowrates ~j stdl must be determined As the magmtudes of the flowrates cannot be measured directly, they must be estimated from concentratmns measured under controlled condmons. As the mterzonal mr flows are independent of the particular contaminant being modeled, a tracer gas can be used Assuming that the outdoors concentraUon of the tracer gas ~s neghg~ble, then equauon (3) becomes V. dC' = q~,-- I~,.C, dt

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In the r o o m where the tracer gas was released, the concentrauon increased to a high level tmmedmtely after the release, then decreased rapidly as the tracer gas moved to other rooms and outdoors. Once all rooms had attained their peak concentratmns, the rates of decrease slowed down In the other rooms, the rise m concentratmn took a longer t~me, and the time increased as the d~stance between the r o o m and the point of release increased In some cases (see F~gs 6 and 7), ~t took up to 5 h for the other rooms to reach their peak concentratmns

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A Method to Select Locations for Indoor Aw Quality Sampling

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This is possible because of the hnear nature of equation (5), which allows for apphcatlon of the prmctple of superposltlon of solutions The use of relatwe exposure mdlces allows for accurate m o d e h n g of relative occupant exposure m the different parts of the bulldmg Unfortunately, the d u r a t m n a n d n u m b e r of tests required for this m o d e h n g are too great for widespread use m the field In the next section, however, a simpler though less accurate m e t h o d will be introduced which wdl be related to the concept of relatwe exposure

The values of the relative exposure . d i c e s obtained in the Energy Research H o u s e are summarized in Table 2 These results show that, ff the relatwe m a g m t u d e s of the sources of a particular c o n t a m m a n t are known, the occupants' relative zonal exposure can be calculated as a hnear combmat~on of the values of the relative exposure index

N Risk = ~ E,t~,

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Table 2 Values of the relative exposure index m the Energy Research House* Zoner SE bedroom SW bedroom N bedroom Lwmg room Basement Upper greenhouse Lower greenhouse Environmental conditions dunng tests Average outdoor temperature (°C) Average mdoor temperature (°C) Average wmd speed and direction (m/s)

Relative exposure mdex, E, for tracer-gas release m Lower greenhouse Basement Lvlng room SE bedroom 1 13 1 24 1 17 0 90 0 91 1 13 1 00

1 01 0 99 0 82 0 95 1 00 0 88 0 82

0 87 0 85 0 95 1 00 0 79 0 85 0 69

0 17 1 00 0 22 0 15 0 09 0 08 0 05

- 7 14 4 3 (S)

- 36 16 4 9 (E)

05 17 3 0 (S)

--9 7 20 1 8 (S)

* The values of E were obtmned by numerical mtegraUon of the curves shown m Figs 5-8 It was also assumed that decay of each mdwldual curve remaaned the same as dunng the last three hours of the tests See [12] for calculation details t Although other zones m the house were also measured, only one of each type of distinct pattern found is shown For more detads, see [12]

E. A. B. Maldonado and J. E. Woods

178

radices and will allow a quick determination of the highrisk zones

Table 3 Ventflatmn efficmnmesm the Energy Research House

MEASUREMENT OF ZONAL VENTILATION EFFICIENCIES As previously discussed, internal air movement and exchange with outdoor air are the major mechamsms responsible for contatmnant dlstnbutlon and dilution Inside a building Thus, the concept of ventllatmn efficiency, l e a measure of the renewal of air at a particular point [13, 14], might be sufficient to quahtatlvely describe contaminant dlstnbutmns m a building In particular, the multlzone testing designated by Sandberg [13] as the'area under the curve' method seems the most appropriate for evaluation of ventilation efficiency for this purpose As gwen by equatmn (1), the 'area under the curve' can be interpreted as a measure of occupant exposure to a contaminant In a test to evaluate ventllaUon efficaency, which is shown in Fig 9 for the Energy Research House, a uniform concentration of tracer gas is established throughout the house (e g by use of the central mr-handhng system or portable fans) and then allowed to decay naturally Zones where more outdoor mr, free of tracer, is introduced will show a faster decay of tracer-gas concentratmn than those zones where a smaller amount of outdoor mr (1 e a smaller ventilation efficiency) is introduced The outdoor mr can be introduced into a particular zone d~rectly from outdoors or through internal air flows with other zones in the building The ventilation efficlencles were calculated as the ratio of the numencally integrated values of the 'area under the curve' for each channel and an area selected as reference

6

Zone

e

SE bedroom SW bedroom N bedroom Llwng room Basement Upper greenhouse Lower greenhouse

0 85 0 82 0 84 l 00 1 06 1 22 1 11

Enwronmental condmons dunng test Average outdoor temperature (°C) Average indoor temperature (°C) Average wind speed and dlrectmn (m/s)

17 19 2 7 (SE)

Table 3 hsts the ventilation efficlencles measured in the test shown in Fig 9, where the living r o o m was chosen as the reference area because of its central locatmn in the building Comparison of Tables 2 and 3 shows that the highest value of the relative exposure index always occurred at the zone where the contaminant (l e tracer gas) was released or in a zone that had a lower ventilation efficmncy than the zone where the tracer was released Moreover, the zones that had similar ventdatlon efficlencles usually also had similar E values It should be noted, however, that when the tracer-gas release occurred in the southeast bedroom, one of the rooms which had the lowest ventilation efficlencles in the house, little transport occurred to other zones because of the limited air movement which is Inherently associated with low ventilation efficlencles Finally, the differences In magmtude of the relatwe exposure index in a test

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A Method to Select Locatwns for Indoor Air Quality Sampling

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increased as the ventilation efficmncy of the zone where the tracer gas was released also increased On the basis of the previous observations it can be concluded that, m most cases, the highest-risk zone is either the location where a contaminant is generated or one of the zones with the lowest ventdatlon efficmncy in the braiding If monitoring is done at those two locations, the highest-risk zone should be one of the momtored zones or another which has concentrations only shghtly higher than the momtored zone (because the values of E are of the same order of magmtude) Thus, only shght errors m estimating risk to the occupants should result if this method Is used

CONCLUSION The rationale presented for the selection of contaminant samphng locations m a building will allow relatwely expe&ent evaluations of indoor air quahty Moreover, this rationale may also be used to develop practical surve3qng techmques m braidings where there are no known indoor air quahty problems A practical techmque for this latter case has been described elsewhere m greater detail [15] It should be noted that infiltration, natural ventdatlon, and mdoor air movement, which were postulated to be the most ~mportant driving mechamsms for indoor contaminant dlstnbut~on, are inherently unsteady phenomena

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E A B. M a l d o n a d o a n d J. E W o o d s

180

Thus, when s a m p h n g for indoor mr quahty, care must be taken to obtain data under weather conditions c h a r a c t e n s h c of the site Similar observations apply to the type of indoor occupancy and c o n t a m m a n t generation patterns

Acknowledgments--Wewould hke to thank the Office of the VicePresident for Research and the Engineering Research Institute at Iowa State University, the Iowa Energy Pohcy Council, the Portuguese Government, and the Luso-Amencan Education Commission for providing the funds which made this study possible

REFERENCES 1 2 3 4

5

6 7 8 9 I0 11 12 13 14 15

T Tredgold, Principles of Warming and Ventdatm0 Public Buildings Taylor, London (1824) J E Woods, E A B Maldonado and G L Reynolds, How ventdatlon influences energy consumption and indoor air quality ASIIRAE J 23, 40--43 (1981) A K Klauss, R H Tull, L M Roots and J R Pfafflm, History of the changing concepts in ventilation requirements ASItRAE J 12, 51-55 (1970) O Valbjorn, P A Nielsen and J Kjoer, Indoor climate problems in Danish dwellings Complaints and diseases referred to the type and materials of dwellings and the use of alrmgs Presented at the International Symposium on Indoor Air Pollution, Health and Energy Conservation, Amherst, Massachussetts (1981) I Andersen, Formaldehyde in the indoor environment--health implications and the setting of standards In Indoor Climate, pp 65-87 Edited by P O Fanger and O Valb.lorn Danish Building Research Institute, Horsholm, Denmark (1978) F J Offerman, C D Hollowell, W W Nazaroff, G D Roseme and J R Rlzzuto, Low-infiltration housing in Rochester, New York a study of air-exchange rates and indoor air quality Envlr lnt 8, 435445 (1982) D Moschandreas (Ed), Indoor Air Pollution m the Res~dentzal Environment 2 Vols U S Environmental Protection Agency Report EPA-600/7-78-229a and -229b U S Government Printing Office, Washington, DC (1978) J B Dick, Measurement of ventdatlon using tracer gas techmque Heat ~Piping~Air Cond 22, 131--137 (1950) E R Hltchln and C B Wilson, A review of experimental techniques for the investigation of natural ventdatlon in buildings Budd Scl 2, 59-82 (1967) D R Bahnfleth, T D Moseley and W S Harris, Measurement of infiltration m two residences ASHRAE Trans 63, 439-476 (1957) R C Jordan, G A Enckson and R R Leonard, Infiltrat,on measurements in two research houses ASHRAE Trans 69, 344-350 (1963) E A B Maldonado, A method to characterize air exchange in residences for evaluation of indoor mr quality Ph D dissertation, Iowa State University, Ames (1982) M Sandberg, What is ventilation efficiency9 Bldg Envir 16, 123-135 (1981) E Skaret and H M Mathisen, Ventilation efficiency Envlr lnt 8, 473-481 (1982) E A B Maldonado and J E Woods, A procedure for field surveys of indoor air quality in energy-efficient residences Proceedings of the Second International Congress on Building Energy Management, Ames, Iowa (1983)

APPENDIX: E X P E R I M E N T A L F A C I L I T Y AND INSTRUMENTATION All the measurements reported here were made m the Iowa State Umverslty Energy Research House, located m Ames, Iowa Its schematic floor plan is shown m Fig 10 Tracer gas (SF6) measurements were made using the eqmpment shown schematically m Fig 11 Twelve sampling channels were

measured m sequence, one at a time on a 12-mm cycle, and the sample collected at one location was returned to the same location via the return mamfold This was done to avoid cross-channel contamination due to the 25 l/mm samphng rate reqmred by the gas analyzer Except when specified differently m the text, the 12 probe locations were as indicated m Fig 10