- Email: [email protected]
Performance of the surface air system air samplers
Journal of Hospital Infection (1985) 6, 102-107
EQUIPMENT REPORT
P e r f o r m a n c e of the surface air s y s t e m air s a m p l e r s V. L a c h
Environmental Microbiology and Safety Reference Laboratory, P H L S CAMR, Porton Down, Salisbury, Wiltshire SP4 0JG Accepted for publication 18 June 1984 Summary: Two models of the surface air system (SAS) air sampler have been compared with the Bourdillon slit sampler. These tests show that the effective sampling volume remains nearly constant with an efficiency approximating to 100% over the range of particle sizes likely to be encountered in environmental sampling. At particle sizes below 4pro the collection efficiency falls off and with 2pm particles the efficiency is reduced to 50~ The high air flows, c. 200 1 min ~of the samplers are particularly suited to measurements of low levels of airborne micro-organisms. Introduction T h e s a m p l i n g of air for the collection, c u l t i v a t i o n a n d e n u m e r a t i o n of a i r b o r n e m i c r o - o r g a n i s m s p r e s e n t s t e c h n i c a l difficulties. T h e s m a l l size a n d c o n s e q u e n t low inertia o f the particles i n v o l v e d w h i c h , for the b a c t e r i a l a n d f u n g a l species, m a y be s o m e w h a t less t h a n 1 btm n e e d s h i g h air velocities or s m a l l p o r e size filters if t h e s e are to b e i s o l a t e d f r o m the air s t r e a m . P u m p s c a p a b l e o f this h a v e b e e n n o i s y a n d b u l k y a n d r e q u i r e d c o n s i d e r a b l e p o w e r . T h e samplers tested here (Pool Bionalyse haliana SAS sampler supplied by C h e r w e l l L a b o r a t o r i e s , Bicester) are i n t e n d e d to p r o v i d e p o r t a b l e selfc o n t a i n e d air s a m p l i n g s y s t e m s . T h e m o s t i m p o r t a n t c h a r a c t e r i s t i c o f an air s a m p l e r is the effective v o l u m e - r a t e o f s a m p l i n g a n d the r a n g e of p a r t i c l e sizes o v e r w h i c h this is m a i n t a i n e d . T e s t s to d e t e r m i n e this w e r e , t h e r e f o r e , c a r r i e d o u t w i t h the surface air s y s t e m ( S A S ) s a m p l e r s .
Description of the samplers T h e s e are single stage sieve t y p e s a m p l e r s ( F i g u r e 1). T h e c o l l e c t i n g s u r f a c e is p r o v i d e d b y a R o d a c plate. T w o m o d e l s are available, o n e w i t h 220 holes uses a 50 m m R o d a c p l a t e c o n t a i n i n g 12 m l agar, the s e c o n d w i t h 260 holes uses a 90 m m R o d a c p l a t e c o n t a i n i n g 13-5 m l agar, b o t h s a m p l e r s o p e r a t e at a q u o t e d flow r a t e of 180 1 r a i n -1. T h e s u c t i o n fan a n d a solid state t i m e r , 0195-6701/85/010102 + 06 $03.00/0
(C) 1985 T h e Hospital Infection Society
102
Performance of air samplers
103
Figure 1. Diagrammatic section of SAS sampler head.
variable from 20 s to 6 min, are built into the sampler body. A 12 V power supply is required, both a mains transformer and a rechargeable pack are available. T h e latter makes this sampler completely portable. Methods
T h e volumetric air flows into the samplers were measured by attaching a 100 m m diameter pipe to the sampler inlet and determining the air velocity down the pipe with a 100 m m vane anemometer (Airflow Developments Ltd). T h e volumetric flows were calculated from the air velocity and the cross section of the pipe. T h e performance of the samplers was determined by comparison with a slit sampler (Bourdillon, Lidwell & T h o m a s , 1941) which has known characteristics. T h e slit sampler operated at 30 1 rain- 1. Both the 50 m m and 90 m m plate SAS samplers were used. (1) Parallel samples were taken at various sites in the laboratory complex. All three samplers were run for 5 rain periods. (2) Parallel 2 rain samples were taken in a small room (about 28 m 3) into which an aerosol of Bacillus globigii spores were liberated from a Collison atomiser operating at 1"8 bar (May, 1973), using a suspension of the spores in distilled water. (3) Test aerosols of a range of controlled sizes were generated in the same room by a spinning disc atomiser (Walton & Prewett, 1949; Foord & Lidwell, 1975). T h e atomiser disc (25 m m in diameter) was driven at 24000 or 48000 r rain- 1and fed with a suspension of B. globigii spores in 80% ethyl alcohol containing varying a m o u n t s of potassium iodide. Solutions
V. L a c h
104
containing 7%, 0"7%, 0"07%, 0"007% and no potassium iodide were used. T h e combination of speed and concentration gave particle diameters of 10, 4"9, 2"3, 1"0 and 0"7 I~m. T h e size of the larger particles was determined microscopically, the smaller by calculation and the 0"7 lim corresponds to a single spore. T h e samplers were run for 2 rain simultaneously with their air intakes about 90 cm above the floor. Results
T h e quoted nominal flow for both samplers was 1801 min -1. Measured values were 2121 min -1 for the 5 0 m m plate model and 2001 min-~ for the 90 m m plate model. T h e results for the sieve samplers must be expressed in terms of + ve, i.e. colony present or - ve, colony absent, for the total n u m b e r of sieve holes. Except at very low levels of airborne particles corrections m u s t be applied for coincidence of two or more colony forming units (cfu) passing t h r o u g h the same hole. T h e results presented here were corrected using the m e t h o d of Andersen (1958), suitable correction charts were supplied with the samplers. (1) T h e results of the environmental samples are given in Table I. (2) T h e results of the tests with single B. globigii spores liberated from a Collison atomiser are given in Table I I. T h e effective sample rate is low for both models of sampler. (3) T h e results of the tests with aerosols of k n o w n size are given in Figure 2. T h e y are expressed as equivalent sampling rate against particle size after correction of the colony counts for coincidence. T h e particle size parameter is the equivalent particle diameter, the diameter of a sphere of unit density having the same settling rate as the potassium iodide particles assuming a single spore to each particle and the density of potassium iodide to be 3 g cc- ~. Each graph point represents the mean of five or more replicate measurements. Discussion
T h e measured air flows of the SAS were some 10-20% greater than the quoted values. T h i s is unlikely to be a significant variation and may well reflect differences in the methods of measurement. T h e results of the environmental samples showed the SAS samplers to have 7(~80% of the efficiency of a slit sampler, rather higher if the rated volume rate rather than the measured rate was used. However, the two types of sampler have markedly different flow rates, 301 rain-1 for the slit sampler and c. 200 1 m i n - 1 SAS. T h e results for the slit sampler are based on very low colony counts, mainly 2-10 colonies, where small variations cause large changes in the results when expressed as cfu m -3
105
? r~
?
i.
o
106
V. L a t h T a b l e II. Bacillus globigii sporesfrom Collison atomiser (mean of 3 replicates) S A S samplers no. of + ve holes
Slit sampler (601 sample)
Theoretical concentration (cfu m -3)
colonies collected
cfu m -3
Background 300 3000
Nil < 17 14 234 116 1940 Effective sample rate* ( l m i n -1)
50mm
90 m m
Nil 4 10 2'6
Nil Nil 4
1.0
* Calculated from the figures in the bottom row.
T h e results of the tests with aerosols of known particle size show that over the range normally encountered in environmental sampling (Noble, Lidwell & Kingston, 1963) the effective sampling rate is, for practical purposes, constant for both models of SAS sampler with an efficiency close to 100% of the slit sampler. T h e effective sampling rate for very small particles is low and this precludes the use of the SAS sampler where single cell aerosols are used, e.g. safety cabinet testing to BS 5726. Overall the performance of both models of SAS sampler are very similar. T h e 9 0 m m model provides more easily read plates at high aerosol concentrations but has the disadvantage of using a n o n - s t a n d a r d plate.
250
20o
o
150
// t
i Q.
~ I00
/
E o
/
Normal range of environmenta[ airborne bacteria (Noble et aZ, ~963)
9 Approximate 5 0 % cut off
r,, 50
0
9
I I-0
1 2'0
] 4'0
I I0
Equivaleparti nt cldeiamet(epr.m)
I 20
40
Figure 2. Effective sample rate vs. equivalent particle d i a m e t e r for two m o d e l s of SAS sampler.
P e r f o r m a n c e of air samplers
107
W h e r e low aerosol c o n c e n t r a t i o n s are to be m e a s u r e d the relatively closer s p a c i n g of the sieve holes in the 50 m m plate m o d e l is u n l i k e l y to p o s e problems. T h e S A $ s a m p l e r s p r o v i d e an easily p o r t a b l e e n v i r o n m e n t a l air s a m p l i n g m e t h o d w h i c h p r o v i d e s essentially u n i f o r m c o l l e c t i o n efficiency o v e r the particle size r a n g e s likely to be e n c o u n t e r e d . References
Bourdillon, R. B., Lidwell, O. M. & Thomas, J. C. (1941). A slit sampler for collecting and counting airborne bacteria. Journal of Hygiene (Cambridge) 41, 197 224. Foord, N. & Lidwell, O. M. (1975). Airborne infection in a fully air conditioned hospital. II Transfer of particles between rooms resulting from the movement of air from one room to another. Journal of Hygiene (Cambridge) 75, 31-44. May, K. R. (1973). The Collison nebulizer; description, performance and application. Aerosol Science 4, 235-243. Noble, W. C., Lidwell, O. M. & Kingston, D. (1963). The size and distribution of airborne particles carrying micro-organisms. Journal of Hygiene (Cambridge) 61,385-391. Walton, W. H. & Prewett, W. C. (1949). The production of sprays and mists of uniform drop size by means of spinning disc type sprayers. Proceedings of the Physical Society 62, 341-350.
EQUIPMENT REPORT
P e r f o r m a n c e of the surface air s y s t e m air s a m p l e r s V. L a c h
Environmental Microbiology and Safety Reference Laboratory, P H L S CAMR, Porton Down, Salisbury, Wiltshire SP4 0JG Accepted for publication 18 June 1984 Summary: Two models of the surface air system (SAS) air sampler have been compared with the Bourdillon slit sampler. These tests show that the effective sampling volume remains nearly constant with an efficiency approximating to 100% over the range of particle sizes likely to be encountered in environmental sampling. At particle sizes below 4pro the collection efficiency falls off and with 2pm particles the efficiency is reduced to 50~ The high air flows, c. 200 1 min ~of the samplers are particularly suited to measurements of low levels of airborne micro-organisms. Introduction T h e s a m p l i n g of air for the collection, c u l t i v a t i o n a n d e n u m e r a t i o n of a i r b o r n e m i c r o - o r g a n i s m s p r e s e n t s t e c h n i c a l difficulties. T h e s m a l l size a n d c o n s e q u e n t low inertia o f the particles i n v o l v e d w h i c h , for the b a c t e r i a l a n d f u n g a l species, m a y be s o m e w h a t less t h a n 1 btm n e e d s h i g h air velocities or s m a l l p o r e size filters if t h e s e are to b e i s o l a t e d f r o m the air s t r e a m . P u m p s c a p a b l e o f this h a v e b e e n n o i s y a n d b u l k y a n d r e q u i r e d c o n s i d e r a b l e p o w e r . T h e samplers tested here (Pool Bionalyse haliana SAS sampler supplied by C h e r w e l l L a b o r a t o r i e s , Bicester) are i n t e n d e d to p r o v i d e p o r t a b l e selfc o n t a i n e d air s a m p l i n g s y s t e m s . T h e m o s t i m p o r t a n t c h a r a c t e r i s t i c o f an air s a m p l e r is the effective v o l u m e - r a t e o f s a m p l i n g a n d the r a n g e of p a r t i c l e sizes o v e r w h i c h this is m a i n t a i n e d . T e s t s to d e t e r m i n e this w e r e , t h e r e f o r e , c a r r i e d o u t w i t h the surface air s y s t e m ( S A S ) s a m p l e r s .
Description of the samplers T h e s e are single stage sieve t y p e s a m p l e r s ( F i g u r e 1). T h e c o l l e c t i n g s u r f a c e is p r o v i d e d b y a R o d a c plate. T w o m o d e l s are available, o n e w i t h 220 holes uses a 50 m m R o d a c p l a t e c o n t a i n i n g 12 m l agar, the s e c o n d w i t h 260 holes uses a 90 m m R o d a c p l a t e c o n t a i n i n g 13-5 m l agar, b o t h s a m p l e r s o p e r a t e at a q u o t e d flow r a t e of 180 1 r a i n -1. T h e s u c t i o n fan a n d a solid state t i m e r , 0195-6701/85/010102 + 06 $03.00/0
(C) 1985 T h e Hospital Infection Society
102
Performance of air samplers
103
Figure 1. Diagrammatic section of SAS sampler head.
variable from 20 s to 6 min, are built into the sampler body. A 12 V power supply is required, both a mains transformer and a rechargeable pack are available. T h e latter makes this sampler completely portable. Methods
T h e volumetric air flows into the samplers were measured by attaching a 100 m m diameter pipe to the sampler inlet and determining the air velocity down the pipe with a 100 m m vane anemometer (Airflow Developments Ltd). T h e volumetric flows were calculated from the air velocity and the cross section of the pipe. T h e performance of the samplers was determined by comparison with a slit sampler (Bourdillon, Lidwell & T h o m a s , 1941) which has known characteristics. T h e slit sampler operated at 30 1 rain- 1. Both the 50 m m and 90 m m plate SAS samplers were used. (1) Parallel samples were taken at various sites in the laboratory complex. All three samplers were run for 5 rain periods. (2) Parallel 2 rain samples were taken in a small room (about 28 m 3) into which an aerosol of Bacillus globigii spores were liberated from a Collison atomiser operating at 1"8 bar (May, 1973), using a suspension of the spores in distilled water. (3) Test aerosols of a range of controlled sizes were generated in the same room by a spinning disc atomiser (Walton & Prewett, 1949; Foord & Lidwell, 1975). T h e atomiser disc (25 m m in diameter) was driven at 24000 or 48000 r rain- 1and fed with a suspension of B. globigii spores in 80% ethyl alcohol containing varying a m o u n t s of potassium iodide. Solutions
V. L a c h
104
containing 7%, 0"7%, 0"07%, 0"007% and no potassium iodide were used. T h e combination of speed and concentration gave particle diameters of 10, 4"9, 2"3, 1"0 and 0"7 I~m. T h e size of the larger particles was determined microscopically, the smaller by calculation and the 0"7 lim corresponds to a single spore. T h e samplers were run for 2 rain simultaneously with their air intakes about 90 cm above the floor. Results
T h e quoted nominal flow for both samplers was 1801 min -1. Measured values were 2121 min -1 for the 5 0 m m plate model and 2001 min-~ for the 90 m m plate model. T h e results for the sieve samplers must be expressed in terms of + ve, i.e. colony present or - ve, colony absent, for the total n u m b e r of sieve holes. Except at very low levels of airborne particles corrections m u s t be applied for coincidence of two or more colony forming units (cfu) passing t h r o u g h the same hole. T h e results presented here were corrected using the m e t h o d of Andersen (1958), suitable correction charts were supplied with the samplers. (1) T h e results of the environmental samples are given in Table I. (2) T h e results of the tests with single B. globigii spores liberated from a Collison atomiser are given in Table I I. T h e effective sample rate is low for both models of sampler. (3) T h e results of the tests with aerosols of k n o w n size are given in Figure 2. T h e y are expressed as equivalent sampling rate against particle size after correction of the colony counts for coincidence. T h e particle size parameter is the equivalent particle diameter, the diameter of a sphere of unit density having the same settling rate as the potassium iodide particles assuming a single spore to each particle and the density of potassium iodide to be 3 g cc- ~. Each graph point represents the mean of five or more replicate measurements. Discussion
T h e measured air flows of the SAS were some 10-20% greater than the quoted values. T h i s is unlikely to be a significant variation and may well reflect differences in the methods of measurement. T h e results of the environmental samples showed the SAS samplers to have 7(~80% of the efficiency of a slit sampler, rather higher if the rated volume rate rather than the measured rate was used. However, the two types of sampler have markedly different flow rates, 301 rain-1 for the slit sampler and c. 200 1 m i n - 1 SAS. T h e results for the slit sampler are based on very low colony counts, mainly 2-10 colonies, where small variations cause large changes in the results when expressed as cfu m -3
105
? r~
?
i.
o
106
V. L a t h T a b l e II. Bacillus globigii sporesfrom Collison atomiser (mean of 3 replicates) S A S samplers no. of + ve holes
Slit sampler (601 sample)
Theoretical concentration (cfu m -3)
colonies collected
cfu m -3
Background 300 3000
Nil < 17 14 234 116 1940 Effective sample rate* ( l m i n -1)
50mm
90 m m
Nil 4 10 2'6
Nil Nil 4
1.0
* Calculated from the figures in the bottom row.
T h e results of the tests with aerosols of known particle size show that over the range normally encountered in environmental sampling (Noble, Lidwell & Kingston, 1963) the effective sampling rate is, for practical purposes, constant for both models of SAS sampler with an efficiency close to 100% of the slit sampler. T h e effective sampling rate for very small particles is low and this precludes the use of the SAS sampler where single cell aerosols are used, e.g. safety cabinet testing to BS 5726. Overall the performance of both models of SAS sampler are very similar. T h e 9 0 m m model provides more easily read plates at high aerosol concentrations but has the disadvantage of using a n o n - s t a n d a r d plate.
250
20o
o
150
// t
i Q.
~ I00
/
E o
/
Normal range of environmenta[ airborne bacteria (Noble et aZ, ~963)
9 Approximate 5 0 % cut off
r,, 50
0
9
I I-0
1 2'0
] 4'0
I I0
Equivaleparti nt cldeiamet(epr.m)
I 20
40
Figure 2. Effective sample rate vs. equivalent particle d i a m e t e r for two m o d e l s of SAS sampler.
P e r f o r m a n c e of air samplers
107
W h e r e low aerosol c o n c e n t r a t i o n s are to be m e a s u r e d the relatively closer s p a c i n g of the sieve holes in the 50 m m plate m o d e l is u n l i k e l y to p o s e problems. T h e S A $ s a m p l e r s p r o v i d e an easily p o r t a b l e e n v i r o n m e n t a l air s a m p l i n g m e t h o d w h i c h p r o v i d e s essentially u n i f o r m c o l l e c t i o n efficiency o v e r the particle size r a n g e s likely to be e n c o u n t e r e d . References
Bourdillon, R. B., Lidwell, O. M. & Thomas, J. C. (1941). A slit sampler for collecting and counting airborne bacteria. Journal of Hygiene (Cambridge) 41, 197 224. Foord, N. & Lidwell, O. M. (1975). Airborne infection in a fully air conditioned hospital. II Transfer of particles between rooms resulting from the movement of air from one room to another. Journal of Hygiene (Cambridge) 75, 31-44. May, K. R. (1973). The Collison nebulizer; description, performance and application. Aerosol Science 4, 235-243. Noble, W. C., Lidwell, O. M. & Kingston, D. (1963). The size and distribution of airborne particles carrying micro-organisms. Journal of Hygiene (Cambridge) 61,385-391. Walton, W. H. & Prewett, W. C. (1949). The production of sprays and mists of uniform drop size by means of spinning disc type sprayers. Proceedings of the Physical Society 62, 341-350.