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Viruses in wastewater aerosols
Environment International, Vol. 7, pp. 35-38, 1982
0160-4120/82/020035-04503.00/0 Copyright © 1982 Pergamon Press Ltd.
Printed in the USA. All rights reserved.
VIRUSES IN WASTEWATER AEROSOLS B. Fattal and B. Teltsch Environmental Health Laboratory, The Hebrew University Hadassah Medical School, Jerusalem, Israel
Agricultural land application of wastewater is becoming a widely used means of dealing with water scarcity and diminishing sources of unpolluted water. One of the most common forms of land application-sprinkler irrigation-poses an environmental problems in that it created wastewater aerosols. Air sampling experiments done at distances of up to 100 meters from wastewater irrigation sprinklers have detected several airborne enteroviruses: echovirus 1, 25, 29; poliovirus II, and coxsackie BI. The levels of airborne viruses found in wastewater aerosols may be potentially hazardous at any level. Since the impact of these findings on public health is not clear, epidemiological investigations by standard techniques should be conducted. It is estimated that the number of viruses isolated in environmental samples may be smaller by one or two orders of magnitude (90°/o-99°70)than their actual number due to the limitations of virus recovery procedures. The unavailability of a reliable indicator microorganism which fulfills all the requirements for a biological indicator also makes it difficult to determine the virus level in the aerosols. Thus the scientific knowledge available today is insufficient to determine with certainty that no public health risk is created by wastewater aerosols. A comprehensive prospective study will, in all likelihood, provide answers to most questions and make it possible to assess more reliably the extent of the health risk due to wastewater aerosols.
Introduction
The concentration o f live microorganisms in aerosol particles is also influenced by biological processes such as die-away or inactivation. Several factors which influence the biological decay rate in the aerosols are particle size (Schaub et al., 1978), meteorological conditions (Taltsch and Katzenelson, 1978), type o f microorganism ( J o h n s o n et al., 1978), and the protective nature o f the surrounding organic matter ( Z i m m e r m a n , 1963).
In light o f the serious p r o b l e m o f water scarcity in nations such as Israel, m a n y p r o g r a m s are u n d e r w a y and others are in planning for extensive exploitation o f urban sewage for agricultural purposes (Feinmesser and Wikinski, 1979). Countries which do not suffer f r o m water shortages also will u n d o u b t e d l y expand their use o f land application techniques as a means for reducing environmental pollution caused by wastewater. H o w ever, one o f the most c o m m o n f o r m s o f land applicat i o n - sprinkler i r r i g a t i o n - poses an environmental p r o b l e m because it creates wastewater aerosols. Other potential sources o f sewage aerosols are treatment plants ( G o f f et al., 1973), spray f r o m polluted seawater (Baylor et al., 1977), and power station cooling towers using wastewater ( A d a m s et al., 1978). Such sewage aerosols contain 1-10 #m airborne particles which can be carried in air streams substantial distances a w a y f r o m the source. The concentration o f such aerosols, which m a y contain a b r o a d spectrum o f enteroviruses, is reduced with time in the a t m o s p h e r e by physical processes o f atmospheric diffusion, gravitational settling and impaction on surfaces.
Viral A e r o s o l s as a H e a l t h Risk Wastewater aerosols created by sprinkler irrigation are a source o f c o n c e r n as a potential risk for field workers and nearby residents. These people are likely to be exposed directly to viral aerosols, t h r o u g h swallowing or inhalation, and indirectly, t h r o u g h c o n t a m i n a t e d clothing, belongings, and f o o d . In recently c o n d u c t e d air sampling experiments, at a distance o f up to 100 m f r o m the wastewater irrigation sprinklers, airborne enteroviruses were detected: echovirus 1, 25, 29, poliovirus II, and coxsackievirus B1. The average concentration in the air was 3.8 x 10 -2 35
36
B. Fattal and B. Teitsch
isolates/m 3 (Table 1). Other investigators have also reported sampling airborne viruses in wastewater-irrigated fields (Johnson et a/., 1978). These findings establish that aerosolization and transport for significant numbers of viruses are caused by sprinkler irrigation with nondisinfected wastewater. According to Plotkin and Katz (1967) one tissue culture infective dose of virus may be sufficient to cause human infection if it is placed in contact with susceptible cells. However, the minimal infective dose has not yet been established for enteroviruses found in aerosols created under field conditions. Therefore, the levels of airborne viruses found in wastewater aerosols may be potentially hazardous at any level. Aerosol particles 1-5 #m in size can be retained deep in the respiratory system of humans; large particles may sink to the gastrointestinal tract (Brown eta/., 1950). Since the impact of these findings on public health is not clear, epidemiological investigations by standard techniques should be conducted. One of the first retrospective epidemiological studies on the possible health risks associated with sprinkler irrigation with wastewater was carried out in Israel (Katzenelson et a/., 1976). In this study, the authors showed that in 77 kibbutzim (agricultural cooperative settlements) practicing primarily sprinkler irrigation with nondisinfected oxidation pond effluent, the incidence of typhoid fever, salmonellosis, shigellosis, and infectious hepatitis was 2-4 times higher than in 130 control kibbutzim not practicing any form of effluent irrigation. However, the researchers agreed that this study could not provide definite proof that the added
health risk in effluent irrigating kibbutzim was associated with the dispersion of pathogenic microorganisms by aerosols, because a number of alternative pathways of infection existed, e.g., direct contact with irrigation workers, their clothing, or irrigated crops. This preliminary survey had methodological constraints. Most important, the investigators based their findings solely on official communicable disease reports which were submitted to the Ministry of Health, and Israeli law does not require listing all the relevant enteric diseases in these reports. Therefore, no conclusions may be made based on this study in general, and on viral disease incidence in particular. Consequently, new research based on medical and environmental data collected directly at each kibbutz is necessary to confirm or rebut the findings of the initial retrospective study. "Such a study was carried out in Israel involving 79 kibbutzim having a total population of 32,672. Medical data was collected from patients' files at each kibbutz clinic according to a list of 24 defined diseases, half of which were "enteric" and the other half "control" (diseases not associated with wastewater). The environmental data was collected by means of a questionnaire filled out on site. Due to various confounding factors and methodological problems inherent in the nature of the aforementioned study, it was felt that definite conclusions as to the health risks associated with effluent utilization should not be drawn and results should be interpreted with caution. Additionally, data from 68 kibbutzim drawn from kibbutz clinic files were compared with reported cases of viral hepatitis, shigellosis and salmonellosis in the
Table 1. Enteric virus levels in wastewater and in sprinkler emission.
Total Enteroviruses in Air Isolates (BGM cells/m~) c
Date o f Sampling
Sampling Distance (In) a
Total Enteroviruses in Wastewater (PFU/I) b
8/6/77 8/6/77 8/6/77 19/7/77 28/8/77 1/9/77 12/9/77 16/9/77 20/9/77 21/9/77 25/9/77 8/8/78 17/8/78
40 40 50 42 36 40 40 70 70 100 100 100 100
6.5 x 102 6.5 x 102 6.5 × 102 1.1 x 101 6.0 × 10° 5.2 x 101 3.0 x 101 1.7 × 104 1.3 x 104 3.3 x 102 8.2 × 104 N.D. d N.D
N.D. N.D. 1.4 x 10 "~ N.D. N.D. 8.2 x 10 -2 2.5 x 10 "2 2.6 x 10 .2 N.D. 4.8 x 10 "~ N.D. 7.9 × 10 -~ 1.0 x 10 -t
Mean and Standard Deviation
64 ± 27
7.5 x 104 ± 2.3 x 104
3.8 x 10.2 ~ 4.7 X 10"~
aAir sampfing was conducted bi~Ween 4 and 6PM at a mean tempeTature o f 28 °C; wind velocity o f 2.5 m/see; and relative humidity of 40%. The sampler used was o f a cyclone scrubber high volume type with a flow rate o f 600 l/rain. bViruses from wastewater were determined by the plaque assay method on BGM cells; results are given as plaque-forming units (PFU) per liter. c Airborne viruses trapped in sampler collecting fluid were assayed by successive passases of inoculated test-tube tissue cultures. Those tubes which showed successive cytopathic effect were considered viral isolates. dN.D. = Not detected.
Viruses in wastewater aerosols
Ministry of Health files. No serious discrepancy was found between the two sources of data. This study did not confirm the extreme excess of viral hepatitis, shigellosis and salmonellosis found in effluent irrigating kibbutzim in Katzenelson et al.'s 1976 study but there were numerous methodological differences between the two studies making a valid comparison difficult (Fattal et al., 1982). A recent review of health effects studies of wastewater aerosols is contained in the proceedings of the EPA Symposium on Wastewater Aerosols & Disease (Pahren and Jakubowski, 1980). The studies reviewed also did not provide clearcut evidence as to the health risks, if any, associated with aerosols in wastewater use."
Criteria and S t a n d a r d s for Viral Aerosols The definitions of criteria and standards call for reliable and efficient quantitative methods to determine virus presence in aerosols. As yet, there are no standard methods available for air sampling or for recovering viruses in the laboratory. Investigators are faced with problems unique to the area of environmental monitoring in general and of biological aerosols in particular. It is estimated that the number of viruses isolated in environmental samples may be smaller by one or two orders of magnitude (90°70-99°7o) than their actual number, due to the limitations of the virus recovery procedures (Akin et al., 1978). Aerosol sampling is a most complex procedure and its efficiency depends on meterological and topographical conditions. The commonly used air sampling techniques do not fulfill the requirements for specific detection and identification of the wide variety of pathogenic viruses liable to be in the aerosol. There is no uniformity in the samplers used nor in the laboratory methods for Virus identification in the collectingliquid. Also, different microorganisms have different degrees of viability in aerosols. The unavailability of a reliable indicator microorganism that fulfills all the requirements for a biological indicator also makes it difficult to determine the virus level in the aerosols. An alternative to the sampling approach is the use of an atmospheric dispersion model to predict the density of any micoorganism to be found in wastewater aerosols. A number of researchers (Camann et al., 1978; Lighthart and Frish, 1976) have proposed such models based on atmospheric pollutant dispersion equations. The models are particularly appropriate for determining the microorganism concentration in the air by means of (a) a parameter that expresses the aerosolization efficiency of the specific sprinkler at each site; or (b) a parameter that expresses the specific decay of each microorganism. With these models it is possible to estimate the density of aerosolized viruses to which workers or residents are exposed in the vicinity of a sprinkler site. However, these calculated estimates can-
37
not be used as evidence in any legal suit since they deal with a hypothetical situation and do not provide direct proof from the site itself. In order to set a standard for viral aerosols it is first necessary to establish some criteria for the threshold level of viruses in the air, a level which would ensure the safety of persons residing or working near the site of aerosolization. The standard which is based on this criterion must also state what steps must be taken to maintain the criterion, i.e., a need to determine a safety zone between the field and the nearby settlement or adjacent path; the necessity for using only treated and disinfected wastewater; or a requirement that only drip irrigation should be carried out with wastewater, thus totally preventing aerosolization. At present, there are no legal standards for virus levels in the air. In the absence of such standards it is extremely difficult for an injured party to prove that his health had been impaired as a direct result of pathogens in the air. When legally set standards do exist and are violated, the presence of a health hazard is presumed without the necessity for legal proof. Therefore, caution should be exercized in determining permissible levels of pathogens in the environment. The scientific knowledge available today is insufficient to determine with certainty that no public health risk is created by wastewater aerosols. Many questions remain as to the degree of possible risk involved. However, there is a good chance that a comprehensive prospective study will provide answers to most of these questions, and thus it will be possible to assess more reliably the extent of the health risk due to wastewater aerosols. Acknowledgments-Special acknowledgements to Mrs. Simona Kedmi of our laboratory for her aid in writing and editing this paper.
References Adams, A. P., Garbett, M., Rees, H. B., and Lewis, B. G. (1978) Bacterial aerosols from cooling towers, J. Water Poll. Cont. Fed., 2362-2369. Akin, E. W., Jakubowski, W., Lucas, J. B. and Pahren, H. R. (1978) Health Hazards Associated with Wastewater Effluents and Sludge: Microbiological Considerations, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludges, B. P. Sagik, and S. A. Sorber, eds. pp. 9-24. University of Texas at San Antonio, San Antonio TX. Baylor, E. R., Baylor, M. B. Blanchard, D. C., Syzdek, L. D., and Appel, C. (1977) Virus Transfer from Surf to Wind, Science 198, 575-577. Brown, J., Cook, K. Ney, F., and Hatch, T. (1950). Influence of particle size upon the retention of particulate matter in the human lung, Am. J. Public Health 40, 450-458. Camann, D. E., Sorber, C. A., Sagik, B. P., Glennon, J. P., and Johnson, D. E. (1978) A model for predicting pathogen concentration in wastewater aerosols, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludges, B. P. Sagik and S. A. Sorber eds. pp. 240-271. University of Texas at San Antonio, San Antonio, TX.
38 Fattal, B., Shuval, H. 1., Wax, Y. and Davies, A. M. (1982) Epidemiological study of disease associated with wastewater utilization in agricultural communities in Israel; Water Reuse Symposium II, Washington, DC (accepted for publication). Feinmesser, A. and Wikinski, M. (1979) Survey of sewage treatment and utilization for agricultural purposes in Israel. Res. Recovery Conserv. 3, 387-392. Goff, G. D., Spendlove, J. C., Adams, A. P., and Nicholes, P. S. (1973) Emission of microbial aerosols from sewage treatment plants that use trickling filters, Health Service Reports 88, 640. Johnson, D. E., Camann, D. E., Sorber, C. A., Sagik, B. P., and Glennon, J. P. (1978) Aerosol monitoring for microbial organisms near a spray irrigation site, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludge, B. P. Sagik and S. A. Sorber eds. pp. 231-239. University of Texas at San Antonio, San Antonio, TX. Katzenelson, F., Buium, I., and Shuval, H. I. (1976) Risk of Communicable disease infection associated with wastewater irrigation in agricultural settlements, Science 194, 944-946. Lighthard, B. and Frish, A. S. (1976) Estimation of viable airborne microbes downwind from a point source, Appl. Environ.
B. Fattal and B. Teltsch Microbiol. 31, 700-704. Pahren, H. and Jakubowski, W., eds. (1980) Wastewater aerosols and disease-Proceedings of a symposium, September 19-21, 1979, EPA 600/9-80-028, U.S. Environmental Protection Agency, Cincinnati, Ohio. Plotkin, S. A. and Katz, M. (1967) Minimal infective doses of viruses for man by the oral route, in Transmission of Viruses by Water Route, G. Berg, ed., p. 151-166. John Wiley and Sons New York. Schaub, S. A., Glennon, J. P., and Bausum, H. T. (1978) Monitoring requirements for land treatment systems, in Proceeding of International Symposium on Land Treatment of Wastewater. Hanover, NH. Teltsch, B. and Katzenelson, E. (1978) Airborne enteric bacteria and viruses from spray irrigation with wastewater. Appl. Environ. Microbiol. 35, 290-296. Teltsch, B., Shuval, H. I., Tadmor, J. (1979) Die-away kinetics of aerosolized bacteria from sprinkler application of wastewater; from Proceedings of Water Reuse Symposium, Vol. 3, pp. 2196-2207, Amer. Water Works Assoc., Denver, Colorado. Zimmerman, L. (1963) Additives for increased aerosol stability, in A Symposium on Airbiology, pp. 285-290. Berkeley.
0160-4120/82/020035-04503.00/0 Copyright © 1982 Pergamon Press Ltd.
Printed in the USA. All rights reserved.
VIRUSES IN WASTEWATER AEROSOLS B. Fattal and B. Teltsch Environmental Health Laboratory, The Hebrew University Hadassah Medical School, Jerusalem, Israel
Agricultural land application of wastewater is becoming a widely used means of dealing with water scarcity and diminishing sources of unpolluted water. One of the most common forms of land application-sprinkler irrigation-poses an environmental problems in that it created wastewater aerosols. Air sampling experiments done at distances of up to 100 meters from wastewater irrigation sprinklers have detected several airborne enteroviruses: echovirus 1, 25, 29; poliovirus II, and coxsackie BI. The levels of airborne viruses found in wastewater aerosols may be potentially hazardous at any level. Since the impact of these findings on public health is not clear, epidemiological investigations by standard techniques should be conducted. It is estimated that the number of viruses isolated in environmental samples may be smaller by one or two orders of magnitude (90°/o-99°70)than their actual number due to the limitations of virus recovery procedures. The unavailability of a reliable indicator microorganism which fulfills all the requirements for a biological indicator also makes it difficult to determine the virus level in the aerosols. Thus the scientific knowledge available today is insufficient to determine with certainty that no public health risk is created by wastewater aerosols. A comprehensive prospective study will, in all likelihood, provide answers to most questions and make it possible to assess more reliably the extent of the health risk due to wastewater aerosols.
Introduction
The concentration o f live microorganisms in aerosol particles is also influenced by biological processes such as die-away or inactivation. Several factors which influence the biological decay rate in the aerosols are particle size (Schaub et al., 1978), meteorological conditions (Taltsch and Katzenelson, 1978), type o f microorganism ( J o h n s o n et al., 1978), and the protective nature o f the surrounding organic matter ( Z i m m e r m a n , 1963).
In light o f the serious p r o b l e m o f water scarcity in nations such as Israel, m a n y p r o g r a m s are u n d e r w a y and others are in planning for extensive exploitation o f urban sewage for agricultural purposes (Feinmesser and Wikinski, 1979). Countries which do not suffer f r o m water shortages also will u n d o u b t e d l y expand their use o f land application techniques as a means for reducing environmental pollution caused by wastewater. H o w ever, one o f the most c o m m o n f o r m s o f land applicat i o n - sprinkler i r r i g a t i o n - poses an environmental p r o b l e m because it creates wastewater aerosols. Other potential sources o f sewage aerosols are treatment plants ( G o f f et al., 1973), spray f r o m polluted seawater (Baylor et al., 1977), and power station cooling towers using wastewater ( A d a m s et al., 1978). Such sewage aerosols contain 1-10 #m airborne particles which can be carried in air streams substantial distances a w a y f r o m the source. The concentration o f such aerosols, which m a y contain a b r o a d spectrum o f enteroviruses, is reduced with time in the a t m o s p h e r e by physical processes o f atmospheric diffusion, gravitational settling and impaction on surfaces.
Viral A e r o s o l s as a H e a l t h Risk Wastewater aerosols created by sprinkler irrigation are a source o f c o n c e r n as a potential risk for field workers and nearby residents. These people are likely to be exposed directly to viral aerosols, t h r o u g h swallowing or inhalation, and indirectly, t h r o u g h c o n t a m i n a t e d clothing, belongings, and f o o d . In recently c o n d u c t e d air sampling experiments, at a distance o f up to 100 m f r o m the wastewater irrigation sprinklers, airborne enteroviruses were detected: echovirus 1, 25, 29, poliovirus II, and coxsackievirus B1. The average concentration in the air was 3.8 x 10 -2 35
36
B. Fattal and B. Teitsch
isolates/m 3 (Table 1). Other investigators have also reported sampling airborne viruses in wastewater-irrigated fields (Johnson et a/., 1978). These findings establish that aerosolization and transport for significant numbers of viruses are caused by sprinkler irrigation with nondisinfected wastewater. According to Plotkin and Katz (1967) one tissue culture infective dose of virus may be sufficient to cause human infection if it is placed in contact with susceptible cells. However, the minimal infective dose has not yet been established for enteroviruses found in aerosols created under field conditions. Therefore, the levels of airborne viruses found in wastewater aerosols may be potentially hazardous at any level. Aerosol particles 1-5 #m in size can be retained deep in the respiratory system of humans; large particles may sink to the gastrointestinal tract (Brown eta/., 1950). Since the impact of these findings on public health is not clear, epidemiological investigations by standard techniques should be conducted. One of the first retrospective epidemiological studies on the possible health risks associated with sprinkler irrigation with wastewater was carried out in Israel (Katzenelson et a/., 1976). In this study, the authors showed that in 77 kibbutzim (agricultural cooperative settlements) practicing primarily sprinkler irrigation with nondisinfected oxidation pond effluent, the incidence of typhoid fever, salmonellosis, shigellosis, and infectious hepatitis was 2-4 times higher than in 130 control kibbutzim not practicing any form of effluent irrigation. However, the researchers agreed that this study could not provide definite proof that the added
health risk in effluent irrigating kibbutzim was associated with the dispersion of pathogenic microorganisms by aerosols, because a number of alternative pathways of infection existed, e.g., direct contact with irrigation workers, their clothing, or irrigated crops. This preliminary survey had methodological constraints. Most important, the investigators based their findings solely on official communicable disease reports which were submitted to the Ministry of Health, and Israeli law does not require listing all the relevant enteric diseases in these reports. Therefore, no conclusions may be made based on this study in general, and on viral disease incidence in particular. Consequently, new research based on medical and environmental data collected directly at each kibbutz is necessary to confirm or rebut the findings of the initial retrospective study. "Such a study was carried out in Israel involving 79 kibbutzim having a total population of 32,672. Medical data was collected from patients' files at each kibbutz clinic according to a list of 24 defined diseases, half of which were "enteric" and the other half "control" (diseases not associated with wastewater). The environmental data was collected by means of a questionnaire filled out on site. Due to various confounding factors and methodological problems inherent in the nature of the aforementioned study, it was felt that definite conclusions as to the health risks associated with effluent utilization should not be drawn and results should be interpreted with caution. Additionally, data from 68 kibbutzim drawn from kibbutz clinic files were compared with reported cases of viral hepatitis, shigellosis and salmonellosis in the
Table 1. Enteric virus levels in wastewater and in sprinkler emission.
Total Enteroviruses in Air Isolates (BGM cells/m~) c
Date o f Sampling
Sampling Distance (In) a
Total Enteroviruses in Wastewater (PFU/I) b
8/6/77 8/6/77 8/6/77 19/7/77 28/8/77 1/9/77 12/9/77 16/9/77 20/9/77 21/9/77 25/9/77 8/8/78 17/8/78
40 40 50 42 36 40 40 70 70 100 100 100 100
6.5 x 102 6.5 x 102 6.5 × 102 1.1 x 101 6.0 × 10° 5.2 x 101 3.0 x 101 1.7 × 104 1.3 x 104 3.3 x 102 8.2 × 104 N.D. d N.D
N.D. N.D. 1.4 x 10 "~ N.D. N.D. 8.2 x 10 -2 2.5 x 10 "2 2.6 x 10 .2 N.D. 4.8 x 10 "~ N.D. 7.9 × 10 -~ 1.0 x 10 -t
Mean and Standard Deviation
64 ± 27
7.5 x 104 ± 2.3 x 104
3.8 x 10.2 ~ 4.7 X 10"~
aAir sampfing was conducted bi~Ween 4 and 6PM at a mean tempeTature o f 28 °C; wind velocity o f 2.5 m/see; and relative humidity of 40%. The sampler used was o f a cyclone scrubber high volume type with a flow rate o f 600 l/rain. bViruses from wastewater were determined by the plaque assay method on BGM cells; results are given as plaque-forming units (PFU) per liter. c Airborne viruses trapped in sampler collecting fluid were assayed by successive passases of inoculated test-tube tissue cultures. Those tubes which showed successive cytopathic effect were considered viral isolates. dN.D. = Not detected.
Viruses in wastewater aerosols
Ministry of Health files. No serious discrepancy was found between the two sources of data. This study did not confirm the extreme excess of viral hepatitis, shigellosis and salmonellosis found in effluent irrigating kibbutzim in Katzenelson et al.'s 1976 study but there were numerous methodological differences between the two studies making a valid comparison difficult (Fattal et al., 1982). A recent review of health effects studies of wastewater aerosols is contained in the proceedings of the EPA Symposium on Wastewater Aerosols & Disease (Pahren and Jakubowski, 1980). The studies reviewed also did not provide clearcut evidence as to the health risks, if any, associated with aerosols in wastewater use."
Criteria and S t a n d a r d s for Viral Aerosols The definitions of criteria and standards call for reliable and efficient quantitative methods to determine virus presence in aerosols. As yet, there are no standard methods available for air sampling or for recovering viruses in the laboratory. Investigators are faced with problems unique to the area of environmental monitoring in general and of biological aerosols in particular. It is estimated that the number of viruses isolated in environmental samples may be smaller by one or two orders of magnitude (90°70-99°7o) than their actual number, due to the limitations of the virus recovery procedures (Akin et al., 1978). Aerosol sampling is a most complex procedure and its efficiency depends on meterological and topographical conditions. The commonly used air sampling techniques do not fulfill the requirements for specific detection and identification of the wide variety of pathogenic viruses liable to be in the aerosol. There is no uniformity in the samplers used nor in the laboratory methods for Virus identification in the collectingliquid. Also, different microorganisms have different degrees of viability in aerosols. The unavailability of a reliable indicator microorganism that fulfills all the requirements for a biological indicator also makes it difficult to determine the virus level in the aerosols. An alternative to the sampling approach is the use of an atmospheric dispersion model to predict the density of any micoorganism to be found in wastewater aerosols. A number of researchers (Camann et al., 1978; Lighthart and Frish, 1976) have proposed such models based on atmospheric pollutant dispersion equations. The models are particularly appropriate for determining the microorganism concentration in the air by means of (a) a parameter that expresses the aerosolization efficiency of the specific sprinkler at each site; or (b) a parameter that expresses the specific decay of each microorganism. With these models it is possible to estimate the density of aerosolized viruses to which workers or residents are exposed in the vicinity of a sprinkler site. However, these calculated estimates can-
37
not be used as evidence in any legal suit since they deal with a hypothetical situation and do not provide direct proof from the site itself. In order to set a standard for viral aerosols it is first necessary to establish some criteria for the threshold level of viruses in the air, a level which would ensure the safety of persons residing or working near the site of aerosolization. The standard which is based on this criterion must also state what steps must be taken to maintain the criterion, i.e., a need to determine a safety zone between the field and the nearby settlement or adjacent path; the necessity for using only treated and disinfected wastewater; or a requirement that only drip irrigation should be carried out with wastewater, thus totally preventing aerosolization. At present, there are no legal standards for virus levels in the air. In the absence of such standards it is extremely difficult for an injured party to prove that his health had been impaired as a direct result of pathogens in the air. When legally set standards do exist and are violated, the presence of a health hazard is presumed without the necessity for legal proof. Therefore, caution should be exercized in determining permissible levels of pathogens in the environment. The scientific knowledge available today is insufficient to determine with certainty that no public health risk is created by wastewater aerosols. Many questions remain as to the degree of possible risk involved. However, there is a good chance that a comprehensive prospective study will provide answers to most of these questions, and thus it will be possible to assess more reliably the extent of the health risk due to wastewater aerosols. Acknowledgments-Special acknowledgements to Mrs. Simona Kedmi of our laboratory for her aid in writing and editing this paper.
References Adams, A. P., Garbett, M., Rees, H. B., and Lewis, B. G. (1978) Bacterial aerosols from cooling towers, J. Water Poll. Cont. Fed., 2362-2369. Akin, E. W., Jakubowski, W., Lucas, J. B. and Pahren, H. R. (1978) Health Hazards Associated with Wastewater Effluents and Sludge: Microbiological Considerations, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludges, B. P. Sagik, and S. A. Sorber, eds. pp. 9-24. University of Texas at San Antonio, San Antonio TX. Baylor, E. R., Baylor, M. B. Blanchard, D. C., Syzdek, L. D., and Appel, C. (1977) Virus Transfer from Surf to Wind, Science 198, 575-577. Brown, J., Cook, K. Ney, F., and Hatch, T. (1950). Influence of particle size upon the retention of particulate matter in the human lung, Am. J. Public Health 40, 450-458. Camann, D. E., Sorber, C. A., Sagik, B. P., Glennon, J. P., and Johnson, D. E. (1978) A model for predicting pathogen concentration in wastewater aerosols, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludges, B. P. Sagik and S. A. Sorber eds. pp. 240-271. University of Texas at San Antonio, San Antonio, TX.
38 Fattal, B., Shuval, H. 1., Wax, Y. and Davies, A. M. (1982) Epidemiological study of disease associated with wastewater utilization in agricultural communities in Israel; Water Reuse Symposium II, Washington, DC (accepted for publication). Feinmesser, A. and Wikinski, M. (1979) Survey of sewage treatment and utilization for agricultural purposes in Israel. Res. Recovery Conserv. 3, 387-392. Goff, G. D., Spendlove, J. C., Adams, A. P., and Nicholes, P. S. (1973) Emission of microbial aerosols from sewage treatment plants that use trickling filters, Health Service Reports 88, 640. Johnson, D. E., Camann, D. E., Sorber, C. A., Sagik, B. P., and Glennon, J. P. (1978) Aerosol monitoring for microbial organisms near a spray irrigation site, in Proceedings of the Conference on Risk Assessment and Health Effects of Land Application of Municipal Wastewater and Sludge, B. P. Sagik and S. A. Sorber eds. pp. 231-239. University of Texas at San Antonio, San Antonio, TX. Katzenelson, F., Buium, I., and Shuval, H. I. (1976) Risk of Communicable disease infection associated with wastewater irrigation in agricultural settlements, Science 194, 944-946. Lighthard, B. and Frish, A. S. (1976) Estimation of viable airborne microbes downwind from a point source, Appl. Environ.
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