(Laboratory of Microbiology, Technical University, Delft, Holland).
S O M E O B S E R V A T I O N S O N A I R F I L T R A T I O N b y
A. J. KLUYVER and J. VISSER
(Received June 17, 1950). 1. INTRODUCTION.AS has already been remarked in our previous communication (1) the problem of the removal of germs from air currents is of ever increasing importance nowadays. It is well-known that the problem is usually solved b y filtering the air through suitable materials. Nevertheless it appears that in industrial practice difficulties are not seldom encountered in this procedure, and therefore it seemed worth-while to collect some data regarding the factors which determine the success of the operation. Although the investigations reported here bear only a preliminary character, some of the results obtained m a y be of sufficient interest to justify their publication.
The more so, since the literature dealing with the subject is very scanty. The only publication known to us which brings some essential d a t a regarding the filtration process is that of TERJESEN and CHERRY (2).
In this s t u d y special attention has been given to the suitability of a special type of slag wool ("Stillite") as a filtering medium. The favourable results obtained with this material induced us to include also "Stillite" in our experiments. On our request the "Stillite Products L t d " 1) kindly put two small sample "Stillite" filter pads at our disposal. Besides this, cotton wool and carbon have been tested on their filtering properties.
2. METHODS AND EQUIPMENT.
In all experiments air was used which had been artificially contaminated with spores of B a c i l l u s cereus applying the modified
312 A . J . Kluyver and J. Visser,
nebulizer described in our preceding communication (l). Hereto no use was made of the secondary air flow through the nebulizer. In the earlier experiments the air was sucked through a bottomless flask which had been filled with the filtering material. The filter as such had previously been subjected to a heat sterilisation at 160 ° C. for one hour. After leaving the neck of the flask the air passed three capillary impingers in parallel arrangement. Each impinger contained as usually 80.mi of water. These impingers together with the connecting tubes had also been previously sterilized in the autoclave at 120 ° C. for 15 minutes.
After the end of the experiment the water in the impingers was tested on the presence of germs either by mixing a suitable q u a n t i t y of the water with melted nutrient agar, or by inoculating 5 × I0 ml, 5 × 1 ml and 5 × 0.1 ml in tubes containing 10 ml peptone water (1 o~o peptone, 0.5 o/o NaC1 in t a p water). After incubation for 16 hours at 37 ° C. colonies on the plates were counted, and when use had been made of the tubes the latter were examined for growth. t~rom the outcome of this examination the most probable number (iXI.N.P.) of bacteria present in the water sample was established with the aid of the table in use for the evaluation of coli tests in water examination (3).
It should be remarked here t h a t in a later phase of our investi- gation we have made some determinations with both plate method and the liquid medium test. IZrom these experiments it appeared t h a t the first method always gave somewhat higher counts. Appar- ently a certain percentage of the spores of B a c i l b t s cereals does not germinate in peptone water under the conditions prevailing in our experiments, whilst t h e y do germinate on the nutrient agar plates. For this reason some reserve must be made with regard to the absolute value of the results reported below; nevertheless it seems acceptable t h a t the total picture is scarcely influenced by this imperfection of the liquid medium test.
In the first experiments the filter was tightly packed with cotton wool, great care being taken to avoid channeling. The results, however, were disappointing, in so far as never complete sterili- zation of the air was a t t a i n e d even with only moderately con- t a m i n a t e d air.
It seemed probable t h a t this unfavourable result would be due to air leakage occurring between the wall of the flask and the cotton wool. For this reason it was decided to improve the filter
S o m e o b s e r v a t i o n s o n a i r f i l t r a t i o n . 31 3
in such a w a y t h a t the danger of edge leaking would be minimized. This led to a construction as reproduced in Fig. I.
In the b o t t o m of a metal cylinder a circular opening of I0 cm d i a m e t e r was made and covered with copper gauze. To this b o t t o m a metal funnel was fixed as indicated in "Fig. 1. Into the cylinder a n u m b e r of discs of cotton wool were brought which then were compressed b y p u t t i n g in a second closely fitting cylinder with broad flanges, also leaving a circular opening of 10 cm diameter. Then a second disc of cotton wool was pressed in the inner cylinder with the aid o f - a n o t h e r closely fitting cylinder with flanges, and this operation was r e p e a t e d for a t h i r d time. The opening of the last cylinder was also covered with copper gauze. All the cylinders were then subjected to strong pressure so that the distance between two flanges was reduced to 9 cm; t h e y were then bolted together.
cotton wool
E
i
v
i / , ° ' inlet |E
i
!Fig. 1. Cotton w o o l filter.
In this way a strongly pressed cotton wool column of 27 cm thick- ness was obtained. It will be clear t h a t leakage of air between cylinder wall and cotton wool can only occur in so far as the air twice passes between the wool and the flanges. Owing to the high pressure the air passage will here be much more difficult t h a n between the wool and the cylinder wall. Moreover, since in each c o m p a r t m e n t the length of direct w a y of the air t h r o u g h the wool will be only 9 cm as compared with a length of 19 cm for the w a y of the air which passes between the wool and the wall, it is ex- t r e m e l y improbable t h a t any leakage will have occurred.
Since it was soon observed t h a t the cotton wool Iost a great deal of its elasticity, if the filter was subiected to a d r y sterilisation, we have later always sterilized the filter in an autoclave at t20 ° C. for 15 minutes. During this operation there was no mechanicaI pressure on the filter.
f
314 A . J . Kluyver and J, Visser,
F o r the stillite filter pads - - each of which measured 30 × 30 × 5 cm - - a special frame was c o n s t r u c t e d of which a sectional
F///////////////////////////////////A
i i
wooden ft'ame siillite pads
rubber sheet
Fig. 2. Stillite filter.
drawing is shown ill Fig. 2. The frame consists of two square metal boxes with conical inlet and outlet for the air. I n t h e inner box a rubber sheet with a circular opening of t0 cm d i a m e t e r , i s brought, and hereupon the two stillite pads rest. These were co- vered b y a n o t h e r rubber
sheet with a similar
opening as the former. T h e n the outer box was put in the position as in- dicated in the figure. In this state the filter was sterilized in an autoclave at 120 ° C. for 15 minutes. Then the boxes were pressed together with the aid of a metal frame bearing four screws. H e r e b y a wooden frame acted as a buffer.
F o r the filtration experiments with carbon we used simple m e t a l cylinders of 15 cm d i a m e t e r which were kept in a vertical position and t h r o u g h which the air passed in downward direction. At t h e b o t t o m of the cylinder there was a copper gauze covered with glass beads, at the t o p of which a n o t h e r gauze acted as a support of the carbon. After some p r e l i m i n a r y experiments the carbon " N o r i t P K 0.5--1 m m " proved to be the most suitable. The carbon filter was ahvays subjected to a d r y sterilisation at 160 ° C. for one hour. In e x p e r i m e n t s with low velocity the c o n t a m i n a t e d air was sucked t h r o u g h the sterilized filters and then passed three capillary impingers in parallel arrangement. In later experiments in which the velocity of the air was raised the limited c a p a c i t y of the im- pingers did only allow the analysis of a fraction of the total flow which should be well kept in mind in considering the results obtained. In these cases we used an arrangement as indicated in 1~ ig. 3.
Some observations on air filtration. 315 cleaner. In order to sample the air after its passage through the filter part of the air flow (9 litre/min.) was sucked through the three impingers with the aid of a vacuum pump. In each of the air flows the rate was determined with the aid of a flowrator of the FISHER and PORTER Cy.
impingerT,~--, - @
',11
i
~'}~----
nebulizer
il
Fig. 3. E x p e r i m e n t a l s c h e m e . f to vacuum cleanerI ~
to vacuum pump
)
flowrator
J
3. E X P E R I M E N T S WITH COTTON \VOOL AS A FILTER MEDIUM.
A first series of experiments was made with air which was contaminated with about 800 spores per litre. Hereto we first determined the amount of spore suspension sprayed b y the nebulizer during one hour. Some experiments had shown that, in accordance with observations of other workers, the number of germs present in the spray is as a rule only about 20 °. o of that calculated on the amount of suspension which disappears from the nebulizer. In the calculation of the contamination we have, therefore, always applied an empirical factor of 0.2. Dependent on the air flow used in each experiment the concentration of the spores in the suspension was
316 A . J . K l u y v e r a n d J. Visser,
varied in such a w a y that always the approximate n u m b e r of 800 per litre was obtained in the air flow which entered the filter.
It seems unnecessary to report here in detail all experiments made. In the beginning it was not rare to find that some spores had passed the filter and were retained in one or more of the impingers. It seems probable that this result was due to unsufficient care in the packing of the cotton wool in the filter.
W h e n gradually more experience had been obtained in this operation these failures disappeared.
After this stage had been attained experiments were made to establish whether raising of the velocity of the air flow would affect the result. Such an experiment is reported in Table I.
It should be remarked once for all that the expression " m a x i m u m linear velocity" in the tables has only an arbitrary significance. The figures given are calculated on the basis of the diameter of the circular opening left b y the flanges present in the filter whilst the space occupied b y the filter material was left out of account. In reality the air velocity in the filter is, therefore, at certain spots m a r k e d l y higher.
Table I.
Experiments on the influence of the velocity of the air flow on the efficiency of a cotton wool filter.
N u m b e r of spores in air before filtration: :k 800 p e r litre; t h i c k n e s s c o t t o n w o o l layer: 27 cm; w e i g h t of c o t t o n wool in filter: 900 g r a m ; d u r a t i o n of each e x p e r i m e n t : o n e hour.
l Vol. of air " M a x . l i n e a r I T o t a l vol. of ' M . P . N . of M . P . N . of N o of passing in v e l o c i t y " of I filtered air spores re- spores pass-
t a i n e d b y
exp. litres]min, air in filter I in litres i ing t h e filter in cm/sec. ! i m p i n g e r s 37a b C d e 9 20 35 51 63 2 4 7 11 14 540 0 1200 0 2100 1 0 3060 t 0 3780 t 0
It appears from Table I that an increase of the air velocity from 9 litre/min, to 63 litre/min, did not affect the satisfactory filtering action. Owing to the rather considerable resistance which the filter
Some observations on air filtration. 31 7 offers to the air passage, it was not feasible to attain a higher velocity with the equipment at our disposal.
It seemed worth-while to repeat these experiments with a more heavily contaminated air flow. This time the contamination was of the order of iO.O00 spores per litre; slight variations in the individual experiments, however, being unavoidable. The results of this series of experiments are given in Table II.
Table II.
Experiments on the efficiency of a cotton wool filter using heavily contaminated air.
Number of spores in air before filtration: ± 2.104 per litre; thickness cotton wool layer: 27 cm; weight of cotton wool in filter: 830 gram.
No of exp. 50a b C d e f 53 5: i , Max.hne-
Duration Vol. of air ar veloci- of exp. passingin t y " of air in hours litres/min, in filter
I ~ in cm/sec. 3/4 3/4 3/4 3/4 3/4 3/4 10 1/3 9 2 20 4 35 7 49 lO 59 12 100 21 81 17 68 14 Total vol. of filtered air in litres 405 900 1575 2205 2655 4500 50220 44880 M.P.N. of spores re- tained by impingers 5 34 M.P.N. of spores passing the filter 2 9 6 0 0 18 43 257
I t is n o t e w o r t h y t h a t in all cases the n u m b e r of spores w h i c h were f o u n d to pass t h e filter is v e r y low in p r o p o r t i o n to t h e millions of spores w h i c h h a v e been r e t a i n e d b y t h e filter. This m a y a c c o u n t for t h e fact t h a t in t w o of t h e e x p e r i m e n t s no spores were t r a p p e d in t h e impingers; o b v i o u s l y a c h a n c e f a c t o r also influences t h e result. T h e r e is no clear i n d i c a t i o n t h a t t h e v e l o c i t y of t h e air influences t h e result.
Nevertheless, t h e d a t a of T a b l e
II,
as c o m p a r e d w i t h those of T a b l e I, leave no d o u b t t h a t a high degree of c o n t a m i n a t i o n of t h e u n f i l t e r e d air presents a real d a n g e r for the success of the filtration operation. T h i s conclusion seems to be w a r r a n t e d , a l t h o u g h it318 A . J . Kluyver and J. Visser,
s h o u l d be t a k e n into a c c o u n t t h a t in this series the c o t t o n wool w a s n o t as t i g h t l y p a c k e d as in t h e f o r m e r series, as m a n i f e s t e d b y t h e h i g h e r v e l o c i t y of air w h i c h could be a t t a i n e d .
T h e fact t h a t in the first series of e x p e r i m e n t s r e p o r t e d in T a b l e I I t h e n u m b e r s of spores r e t a i n e d b y the i m p i n g e r s were v e r y
low,
m a d e it seem desirable to test w h e t h e r these results h a d real significance or t h a t t h e y were o n l y due to i m p e r f e c t i o n s in the t e c h n i q u e used. F o r this reason also t w o e x p e r i m e n t s of longer d u r a t i o n were m a d e , t h e results of which leave no d o u b t t h a t indeed the filter did not r e m o v e all g e r m s from t h e passing air. 4. EXPI~.RI.MENTS \VITH STILI.ITE AS A FILTER ME1)IUM,A m i c r o s c o p i c c x a m i n a t i o u of the stillitc fibres r e v e a l e d their e x t r e m e fineness. A c o m p a r i s o n with c o t t o n wool led to the h)llowing figures:
V a r i a t i o n P r e v a i l i n g diam. i n . diam. in . Stillite 2 . 5 - - 2 5 2 . 5 - - 1 0 C o t t o n w().l 1 5 - - 2 6 1 8 - - 2 0
i"or the e x p e r i m e n t s the stillite filter as described in section 2 has t): en used.
TLc results of a series of e x p e r i m e n t s in which t h e air cCmtami- T a b l e I I I .
E x p e r i m e n t s oi1 the influence of the v e l o c i t y of the air flow on the efficiency of a stillite filter.
Number of spores in air before filtration: ± 800 per litre; thickness stillite layer: 10 cm; duration of each experiment: one hour.
. . . i . . . .
"Max.inear M.P.N. of
No of Vol. of air L velocity" of Total vol. of spores re- M.P.N. of passing in filtered air spores pass- exp. litres/min, air in filter tained by
in cm/sec, in litres impingers " ing the filter 58a b C d e f g h 9 2 540 0 0 20 4 1200 0 O 35 7 2100 0 0 43 .q 2580 2 10 77 16 4620 2 17 102 22 6120 0 0 J 25 27 7500 0 0 153 32 9 1 8 0 0 0
S o m e o b s e r v a t i o n s o n a i r f i l t r a t i o n . 3 1 9 nation was again reduced to about 800 spores per litre are reported in Table III. In this series a n o t h e r a t t e m p t was made to s t u d y the influence of the air velocity on the filtration efficiency.
On the whole the results obtained with the stillite filter are most satisfactory. However, in both experiments 58 d and 58 e one of the 15 tubes containing 10 ml of the impinger fluid led to a develop- ment of B a c i l l u s cereus. The possibility cannot be excluded t h a t in these cases we have been dealing with a stray infection. This seems the more likely because with higher velocities of the air flow no spores were detected in the impingers.
The most gratifying result of the experiments in question is u n d o u b t e d l y the fact that the stillite pads combine a high filtration efficiency with a low resistance to the passing air. This is clearly manifested b y the high air velocities which could be attained with our limited resources, velocities which are approaching those used in technical filter systems (linear velocity: 100 cm see.).
We then decided to establish also for the stillite filter in how far a higher degree of contamination of the air w,mld influence the filtration effect (Table IV).
Table IV.
E x p e r i m e n t s on the efficiency of a stillite filter using heavily c o n t a m i n a t e d air. N u m b e r o f s p o r e s in a i r b e f o r e f i l t r a t i o n : 2.10 4 p e r litre; d u r a t i o n o f e a c h e x p e r i m e n t : o n e h o u r . T h i c k - ! [ " M a x . l i n e - t I h e s s o f } VoI ~ff a i r l a r v e l o c i - T o t a l v o l . , M . P . N . o f . M . P . N . o f N o o f ! s t i l l i t e '; p a s s i n g i ~ t t v " o f a i r ~f f i l t e r e d s p o r e s re- ~ s p o r e s s e x p . l a v e r in " t i t r e s / m i n , t in f i l t e r ' , " a i r in t a i n e d b y p a s s i n g c m ] in c m / s e c , l i t r e s i m p i n g e r s t h e f i l t e r 59 5 t-;f}a 10 b 10 9 "2 49 l~l 165 35 540 2940 70 71) 61 332 4 73
"file results reported in Table IV show conclusively that for the stillite filter too there nit:st be a critical degree of contamination which m a y not be surpassed without endangering the filtration effect.
320 A . J . Kluyver and J. Visser,
U s i n g t h e o n l y s l i g h t l y c o n t a m i n a t e d l a b o r a t o r y air we h a v e been able to e s t a b l i s h t h a t a stillite filter acts also v e r y s a t i s f a c t o r i l y in an e x p e r i m e n t of l o n g d u r a t i o n . W i t h an air v o l u m e of 153 l i t r e s / m i n , passing (linear v e l o c i t y 32 cm/sec.) no g e r m s were d e t e c t e d in the i m p i n g e r s even a f t e r 12 h o u r s filtration.
5. EXPERIMENTS WITH CARBON AS a FILTER MEDIUM.
I n all e x p e r i m e n t s r e p o r t e d in this section use has been m a d e of N o r i t P K 0 . 5 - - 1 ram, this b e i n g a g r a d e of c a r b o n a n s w e r i n g t h e r e q u i r e m e n t u s u a l l y m a i n t a i n e d for c a r b o n used in t e c h n i c a l air f i l t r a t i o n o p e r a t i o n s ,
",'iz.,
t h a t of p a s s i n g a sieve of t5 m e s h (inch) a n d being r e t a i n e d b y a sieve of 30 m e s h (inch).I n t h e first series of e x p e r i m e n t s use was m a d e of h e a v i l y con- t a m i n a t e d air. S t a r t i n g w i t h a c a r b o n c o l u m n of 53 c m it was f o u n d t h a t a g o o d f i l t r a t i o n effect w a s o b t a i n e d . H o w e v e r , u n d e r these c o n d i t i o n s t h e c a p a c i t y of the filter was v e r y low, a n d did n o t exceed 18 litres/min. (linear v e l o c i t y 2 cm/sec.). F o r this reason
T a b l e \:.
The influence of t h e height of the c a r b o n c o l u m n on its filtration efficiency ( h e a v i l y c o n t a m i n a t e d air).
Number of spores in air before filtration: i 5-10 l per litre; duration of each experiment one hour, unless otherwise indicated.
- - - . . . . i . . . :
Height of " !TotaI vol. of Vot. of air ar veloci- ! of filtered
No of exp. 36 39a b 401 ) 42 442 ) 47a b C carbon c o l u m n in cm passing in i t y " of air air i l l litres/min, t in filter [ litres I in cm/sec, i I 53 9 53 9 53 18 29 30 16 31 16 37 8 9 8 20 8 30 spores re- tained by impingers 270 0 540 0 1080 0 3600 0 1860 0 5920 7 540 391 1200 429 1800 555 M.P.N. of spores passing the filter 0 0 0 0 0 29 391 953 1830 1) Duration: 2 hours.
S o m e o b s e r v a t i o n s o n a i r f i l t r a t i o n . 321 a series of experiments was made in which the height of the carbon column was gradually decreased. The results of these experiments are collected in Table V.
The figures in Table V show clearly t h a t under the conditions of the experiment a column height of 16 cm presents more or less a critical value for the filter efficiency, in an experiment of longer duration the filter failed to retain all spores.
It seemed worth-while to check in how far with less contaminated air lower carbon columns - - offering the advantage of higher filtration capacity - - would prove to be acceptable from a filtration point of view. The results of these experiments are given in Table VI. For each of the heights tested the highest velocity mentioned in the tables is the m a x i m u m which could be attained with the aid of our equipment.
Table VI.
The influence of the height of the carbon column on its filtration efficiency (moderately contaminated air).
N u m b e r of s p o r e s ill air b e f o r e f i l t r a t i o n : ~ 800 p e r litre; d u r a t i o n o[ e a c h e x p e r i m e n t : o n e hour. H e i g h t of No of c a r b o n e x p . ! c o l u n l n in c m 52a b c 54a b C d 49a b C d 51a b C d e ! " M a x . l i n e - Vol. of air :, a r veloci- p a s s i n g in I t y " of air l i t r e s / m i n , in filter , in c m / s e c . 16 9 16 20 16 31 11 9 11 20 1~ 30 11 40 8 9 8 20 8 32 8 37 8 9 8 20 8 30 8 40 8 51 1 2 3 1 2 3 4 5 1 2 3 5 4 1 2 3 4.5 6 T o t a l vol. of f i l t e r e d air in litres 540 1200 1860 540 1200 1800 2400 540 1200 1920 2220 540 1200 1800 2400 3060 M . P . N . of s p o r e s re- t a i n e d b y i m p i n g e r s 0 0 0 0 0 0 0 0 0 0 0 22 42 35 50 60 M . P . N . of s p o r e s p a s s i n g t h e filter 0 0 0 0 0 0 0 0 0 0 0 22 93 117 222 340
322 A . J . Kluyver and J. Visser,
If we compare the results presented in Table V and Table V[ we m a y conclude t h a t the filtration effect is, indeed, dependent on the degree of c o n t a m i n a t i o n of the air to be filtered. We see further t h a t u n d e r the changed conditions a height of 8 cm is more or less
a critical value: in tile e x p e r i m e n t s 49 a - - d still a satisfactory filtration took place, but in the e x p e r i m e n t s 51 a - - e , in which a p p a r e n t l y tile cart)on column was somewhat tess compact - - as judged b y the higher m a x i m u m velocity - - the filter allowed m a n y spheres to pass. The l a t t e r experiments also show that the most probable n u m b e r of spores which pass the filter increases with increasing velocity of the air: in first ai)proximation in all cases a constant n u m b e r of st)ores is carried b y a certain volume ~f air. Such a simple relation has not been encountered in the cotton wool and stillitc experiments.
It seems justified to conclude from all experiments made that carNm presents a v e r y efficient filter medium, but has the dis- a d v a n t a g e of a relatively low filter capacity per unit of filter surface. It should, however, not be l~st sigl:t of t h a t this disadvantage is not of an absolute character: it can be counterbalanced either 1) 3 , increasing the filter surface or by raising the pressure of the inlet air. In a final e x p e r i m e n t we have a t t e m p t e d to get some insight into the depth in which spores p e n e t r a t e d into the carbon column. Through a carbon column ~f 16 cm height air c o n t a m i n a t e d with 10.000 spores per litre was sucked with a velocity of 56 litres rain. The e x p e r i m e n t was continued for one hour; the analysis of the three impingers which were connected during the e x p e r i m e n t showed t h a t the air which passed the filter did not contain a n y spores.
After the e x p e r i m e n t was ended a standardized sample was aseptically taken from the carbon column at various depths. This was accomplished by sucking away after each sampling a layer of 2 cm of the carbon with the aid of the v a c u u m cleaner. Each sample was brmlght into a sterile test tube containing 5 ml of sterile water and the tubes were then e v a c u a t e d in order to remove the air.
After thorough shaking the numbers of the bacteria present in the carbon suspensions were d e t e r m i n e d with the Lid of the plate method; dilutions being made where necessary.
Although no absolute value can be a t t r i b u t e d to the figures obtained, t h e y still give a good idea of the distribution of the spores retained in the filter column.
S o m e o b s e r v a t i o n s on air filtration. :~2:~ D e p t h s in c a r b o n c o l u m n N u m b e r of s p o r e s 0 1 l 8,000 2 ,q.500 4 21S 6 t} 8 t1 1 0 0 12 0 1 4 o
In this experiment the spores had a p p a r e n t l y not vet reached a d e p t h of 6 cm; this value is in fair agreement with our earlier experiment 47 in which a carbon column of 8 cm just iailed to retain all spores.
S u m m a r y .
1. A m e t h o d has been developed for testing the filtration efficiency of some filter materials. For each of the materials in- vestigated - - cotton wool, stillite and carbon - - a suitable filter has been devised.
2. The filtered air was analyzed as to its germ content with the aid of a set of 3 capillary impingers.
3. The cotton wool filter gave on the whole satisfactory results provided t h a t due attention was given to the packing of the filter and its sterilisation. Clear indications were obtained that the degree of the contamination of the air was of vital importance.
4. The stillite filter proved to have the advantage of combining a high filtration efficiency with a tow resistance to the passing air. Also for the stillite filter a critical degree of contamination of the air was established; on surpassing this degree the filtration effect was endangered.
5. The carbon filter proved to be most efficient, but had a relatively tow specific filteril~g capacity. It was found that the filtration result was depending on the height of the carbon column and on the velocity and the degree of contamination of the air.
6. It should be stressed that in all experiments artificially con- t a m i n a t e d air was used, and that the number of germs present in the air to be filtered was in all cases m a n y times larger than that usually occurring in normal air.
T h e a u t h o r s w i s h to e x p r e s s t h e i r sincere t h a n k s t o t h e ,,Delftse H o g e - s e h o o l f o n d s " for t h e g r a n t which h a s e n a b l e d t h e m t o c a r r y o u t t h i s in- v e s t i g a t i o n .
324 A . J . K l u y v e r and J. Visser. R e f e r e n c e s .
1. A. J. I~LUYVER and J. VISSER, The d e t e r m i n a t i o n of micro-organisms in air. Antonie van Leeuwenhoek 16, 299, 1950. - 2. S. G. TERJESEN and G. B. CHERRY, The removal of micro-organisms from air by filtration. Trans. Inst. Chem. Eng. N o r t h Western Branch, Manchester, October 1947. - 3. S t a n d a r d Methods for t h e E x a m i n a t i o n of \ ¥ a t e r and Sewage. New York, 9th Ed., 1946, p. 205.