Document(s) 7 of 19

¹Hebrew University of Jerusalem, Israel
²Bar Ilan University, Ranut Gan, Israel

Abstract

This study aimed to develop a risk-assessment/cost-effectiveness approach, to compare the risks of irrigating with wastewater treated to meet various recommended microbial guidelines – World Health Organization (WHO) versus United States Environmental Protection Agency (USEPA) – for unrestricted use in agriculture with the risk of irrigating with untreated wastewater. According to the authors’ estimates, the annual risk of contracting infectious diseases including typhoid fever, rotavirus infection, cholera and hepatitis A from eating raw vegetables irrigated with untreated wastewater is in the range of 1.5 × 10^-1 to 5 × 10^-2, or 5–15% of consumers eating such vegetables will develop a case of disease compared to 10^-6 (0.0001%) of those eating vegetables irrigated with treated wastewater effluent that meets the WHO guideline of 1000 faecal coliforms (FC)/100 ml. The USEPA considers a 10^-4 (0.01%) annual risk of becoming ill with an infectious disease acceptable for drinking water. Cost-effectiveness analysis shows that, on average, in a city with a population of one million, the prevention of a single case (out of 61 cases/year) of the four diseases: hepatitis A, rotavirus infection, cholera and typhoid according to WHO guidelines versus USEPA guidelines would entail an extra annual expenditure of wastewater treatment of US$450,000/case. It is questionable if this is a cost-effective or reasonable public health expenditure. The authors estimate that if every one of a million people ate raw vegetables irrigated with untreated wastewater, there would be a 1 in 10 annual risk (100,000 cases/year) of contracting one of these four diseases. Thus, in the authors’ view irrigating vegetables eaten with raw untreated wastewater presents an unreasonably high health risk. However, treatment to meet WHO guidelines would cost US$125/case prevented. This appears to be reasonably cost-effective, but, is a question that must be decided upon by each community. Evaluating health risks by disability adjusted life years (DALY) is also considered.

Introduction

This study aimed to further develop a risk-assessment approach based on a mathematical model and experimental data, in order to conduct a comparative risk analysis of the various recommended wastewater irrigation microbial health guidelines for unrestricted irrigation of vegetables normally eaten raw (uncooked) based on the initial study by Shuval et al., 1997. The guidelines evaluated were those recommended by WHO (1989) and USEPA/USAID in 1992. Consideration was also given to the implications of irrigating such crops with untreated (raw) wastewater as discussed in other chapters of this volume.

Regulations to protect the health of people who consume crops irrigated by wastewater were initiated by the California State Board of Health. In 1933 they established the first microbial effluent standard that was equivalent to the one required for drinking water, which was then set at a most probable number (MPN) of 2.2 faecal coliforms (FC)/100 ml (Ongerth and Jopling, 1977). However, this standard was difficult to achieve even in developed countries, and was not feasible for most developing countries. In fact, hundreds of cities in the developing world could not afford to meet the very rigorous standards that they had innocently copied from the United States, and, thus, did not build any appropriate wastewater treatment plants.

In 1982 the World Bank and the World Health Organization embarked on a broad-spectrum, multi-institutional scientific study involving three independent teams of scientists to review the available epidemiological and technological evidence on health risks associated with wastewater irrigation (Shuval et al., 1986; Feachem et al., 1983; Struass and Blumenthal, 1989). These studies resulted in the publication in 1989, World Health Organization (WHO) Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture. Based on the new epidemiological and technological evidence, the guidelines recommended a mean of 1,000 FC/100 ml and less than one helminth egg per litre of effluent, for the wastewater irrigation of vegetables eaten raw. The new guidelines have become widely accepted by international agencies including the Food and Agriculture Organization of the United Nations (FAO), United Nations Development Programme (UNDP), United Nations Environment Programme (UNEP) and the World Bank and have been adopted by French health authorities and the governments of a number of developing, and developed countries.

In 1992 USEPA together with USAID published their own guidelines for water reuse. These were primarily intended for use within the USA, but were also developed so that they could be used as guidelines by USAID missions working in developing countries. These new guidelines for the irrigation of crops eaten raw are even stricter than the original California standards and call for no (zero) detectable FC/ 100 ml, biochemical oxygen demand (BOD) of 10 mg/l or less, turbidity of 2 nephalometric turbidity units (NTU) or less and chlorine residual of 1 mg/l. In addition, the guidelines stipulate rigorous engineering requirements for biological treatment, sand filtration, chemical disinfection and various fail-safe redundancies and back-up equipment facilities. The standard of zero detectable FC/100 ml had become the current American drinking water standard, so that once again United States thinking was apparently based on a zero indicator organisms or ‘no risk’ concept, regardless of its technical feasibility and cost-effectiveness for other parts of the world.

Risk-assessment Model

The risk-assessment model developed by Haas et al. (1993) for estimating the risk of infection and disease from ingesting microorganisms in drinking water, was used in this study. However, certain modifications were required to fit the risk of infection associated with eating vegetables irrigated with wastewater of variable microbial quality (Shuval et al., 1997). The probability of infection (P_I ) from ingesting pathogens in water, according to Haas et al. (1993), is presented in Equation 1:

Since not every person infected by the ingestion of pathogens becomes ill, an independent estimate is made of P_D – the probability of ontracting a disease (see Equation 2):

The Number of Pathogens Ingested

Based on laboratory determinations, the authors found that the amount of wastewater of varying microbial quality that would cling to the external surface of wastewater-irrigated cucumbers is 0.36 ml/100 g (one large cucumber) and 10.8 ml/100 g of long-leaf lettuce (about 3 lettuce leaves) (Shuval et al., 1997). Based on these measurements, the amount of indicator organisms that might remain on the vegetables if irrigated with untreated wastewater (with 10⁷ FC/100 ml) and with wastewater meeting WHO guidelines (10³ FC/100 ml) were estimated. According to Schwartzbrod (1995), the ratio of enteric virus : FC is 1:10⁵. For the preliminary risk estimate, it was assumed that all of the enteric viruses are a single pathogen species, such as the viruses of hepatitis A or poliomyelitis, therefore certain assumptions as to median infectious dose and infection to morbidity ratios need to be made.

It was also assumed that under actual field conditions there would be a certain degree of indicator and pathogen die-away and/or removal from the wastewater source until final ingestion by the consumer at home. Factors affecting die-away include: settling, adsorption, desiccation, biological competition, UV irradiation from sunlight, and a degree of removal and/or inactivation as a result of washing the vegetables at home. A number of studies have indicated that there is a rapid die-away or removal of both bacterial indicator organisms and of pathogenic bacteria and viruses in wastewater-irrigated soil and on crops of as much as 5-log in 2 days under field conditions (Bergner-Rabinowitz, 1956; Rudolfs et al., 1951; Sadovski et al., 1978; Armon et al., 1995). Asano and Sakaji (1990) determined virus die-away under field conditions of wastewater reuse, and found that within 2 weeks total virus inactivation reaches about 99.99%, while in 3 days there is a 90% reduction in virus concentration. Even superficial washing of vegetables at home can remove an additional 99–99.9% of the viral contamination. Schwartzbrod (1995) estimated that there would be as much as a 6-log reduction of virus concentration between irrigation with wastewater and consumption of the crops if the total elapsed time reached 3 weeks. To be on the conservative side, it was estimated that the total entero-viruses and bacteria inactivation and/or removal from the wastewater source until ingestion, results in a reduction in pathogenic microorganism concentration by 3-log, or 99.9%, although a 99.99% loss is not unreasonable and might occur in most cases. It can be assumed that this also applies in the case of irrigation with untreated wastewater.

Estimates of Risk of Infection and Disease

Based on the above tests and assumptions, the number of pathogens ingested by a person who eats a 100-g cucumber or 100 g (three leaves) of long-leaf lettuce irrigated with wastewater of various quality was estimated. Four pathogens were selected: two enteric viruses (rotavirus and hepatitis A) and two enteric bacteria (Vibrio cholera and Salmonella typhi), with epidemiological evidence indicating the possibility of their being environmentally transmitted and/ or waterborne (Schwartzbrod, 1995). It was assumed that a minimal infectious dose for 50% of the exposed population to become infected (N₅₀) ranges between 5.6 and 10⁴ depending on the pathogen (see Tables 5.1a and b). While the authors are fully aware that the ratio of infection to clinical disease is often as low as 100:1, they assumed conservatively for this study, that 50% of those infected will succumb to clinical disease (P_D:I = 0.5). They also assumed, based on vegetable consumption patterns in Israel, that on an annual basis a person would consume 100 g of lettuce or cucumbers/day for a total of 150 days. The risk was calculated, using both a severe α value of 0.2, rather than 0.5 (Tables 5.1 and 5.2). However, if α = 0.5 were used it would decrease the risk by about 1-log.

First, as a positive control test of the model the risk of infection and disease from consuming vegetables irrigated with untreated wastewater with an estimated initial FC level of 10⁷/ 100 ml. Assuming a 3-log die-away prior to consumption of the vegetables, it was estimated that under such conditions a 100-g cucumber or 100 g of lettuce irrigated with untreated wastewater would have a final FC level of 30 to 10³. Based on this FC level and a virus:FC ratio of 1:10⁵, there is a probability that when irrigated with untreated wastewater, 3 out of 10,000 cucumbers and 3 leaves of lettuce in 1000 would carry a single enteric virus. According to these estimates of pathogen ingestion, it was estimated that the risk of infection and disease that might result from irrigating lettuce with raw untreated wastewater would vary between 1.5 × 10^-1 and 5 × 10^-2 or 5–15%/year for each of the four diseases studied, with a total of 40% of the population becoming ill with these four diseases each year. To remain on the cautious and conservative side annual total disease risk of some 20% for a range of vegetable crops irrigated with untreated wastewater was assumed. Table 5.1a presents the estimated risk of irrigating lettuce with untreated wastewater, which is a higher than that for cucumbers.

Table 5.1. The risk of infection and disease caused by various pathogens from:

a. Eating 100 g (3 leaves) of long-leaf lettuce irrigated with untreated wastewater once or for 150 days/year.

b. Eating 100 g (3 leaves) of long-leaf lettuce irrigated with treated wastewater effluent meeting the WHO guidelines for unrestricted irrigation of vegetables (1000 FC/100 ml) once or for 150 days/year.

^a Number of pathogens that infect 50% of the exposed population
^bα = 0.265
^cα = 0.2

However, if the effluent is treated to meet the WHO guidelines of 1000 FC/100 ml for irrigation of vegetables to be eaten raw, the risk of infection and disease estimates for lettuce are those shown in Table 5.1b. The risk assessment of consuming 100 g cucumbers irrigated with effluent meeting the WHO guidelines for V. cholera is 10^-9 for a one-time risk of infection or disease, whereas in the case of lettuce it is approximately 10^-7 (Table 5.2). The annual risk of V. cholera from eating lettuce is between 10^-5 and 10^-6.

Table 5.2. The risk of infection and disease caused by Vibrio cholera from eating 100 g of cucumbers or 100 g of long-leaf lettuce irrigated with untreated or treated wastewater effluent meeting the WHO guidelines for unrestricted irrigation.

^a N₅₀ = 10³ and α = 0.2
^b Treated according to the WHO guidelines of 1000 FC/100 ml.

Is this a high- or low-risk level? To shed some light on what are considered reasonable levels of risk for communicable disease transmission from environmental exposure it should be noted that the USEPA has determined that guidelines for drinking water microbial standards should be designed to ensure that human populations are not subjected to a risk of infection by enteric disease greater than 10^-4 (or 1 case per 10,000 person/year) (Regli et al., 1991). Thus, compared with the USEPA estimates of reasonable acceptable risks for waterborne disease-associated microbes ingested directly in drinking water, the WHO wastewater reuse guidelines appear to be some one or two orders of magnitude more rigorous, if not more.

Validation of the Model

a. The 1970 cholera outbreak in Jerusalem

In 1970 an outbreak of cholera involving some 200 cases of clinical disease occurred in Jerusalem. Our investigation and analysis provided strong evidence that the main route of transmission was through the consumption of vegetables, including lettuce and cucumbers, illegally irrigated with untreated wastewater from Jerusalem, which villagers sold door-to-door throughout the city (Fattal et al., 1986). Since considerable and detailed data pertaining to that epidemic were available, it provided an opportunity to test and validate the risk-assessment model against the actual data. Based on microbial tests carried out during the epidemic and other studies, it was estimated that the concentration of cholera vibrios in the raw municipal wastewater was 10–10⁴/100 ml. It was also assumed, based on the literature (Feachem et al., 1983), that the (N₅₀) for cholera in Jerusalem under conditions of good health and nutrition was 10³ vibrios. Table 5.2 shows the theoretical risk of infection and disease from cholera, based on the risk-assessment model. The total number of cases of disease reported in Jerusalem was 200 and it was estimated that some 100,000–200,000 persons purchased the contaminated vegetables and were exposed to the pathogen. Thus, it can be estimated that the case rate in Jerusalem was in the order of 10^-3–10^-4, which falls within the range of the theoretical risk of disease of some 10^-3–10^-5 from lettuce and cucumbers irrigated with untreated wastewater calculated according to the risk-assessment model. It can also be assumed that had the Jerusalem wastewater been treated according to WHO guidelines, the risk of disease transmission by wastewater irrigation would essentially have been negligible, even if the concentration of cholera vibrios in the untreated wastewater had reached the levels it did during the epidemic.

b. The typhoid fever outbreaks in Santiago, Chile, 1978 and 1983

Shuval (1993), who investigated the typhoid fever outbreaks in Santiago in 1978 and 1983, claimed that the use of untreated wastewater for the irrigation of 13,500 ha of various vegetables (tomatoes, lettuce, cabbage, celery, cauliflower), that were consumed raw, was responsible for the transmission of this disease and its high infection rate (~200 cases/100,000 residents). As can be seen in Table 5.1, the one-time risk of becoming ill from S. typhi infection due to the consumption of lettuce irrigated with untreated wastewater is 3.1 × 10^-3. The number of cases of both cholera and typhoid fever predicted by this assessment model is validated by the numbers of actual cases in Jerusalem and Santiago. According to this model, if the wastewater in Jerusalem and in Santiago had been treated according to WHO guidelines (1000 FC/100 ml), the risk of cholera or typhoid infection as a result of eating lettuce irrigated with untreated wastewater would have been very small. The risk run by eating tomatoes or cucumbers would have been negligible.

Cost-effectiveness Analysis

The cost-effectiveness associated with meeting the various wastewater effluent guidelines was estimated. As an example, the hypothetical case of a city in a developing country with a population of one million where currently large areas of vegetable crops are irrigated with untreated wastewater is presented. It is assumed that the city is considering the construction of a wastewater treatment plant to ensure safe utilisation of the effluent for agricultural irrigation of vegetable crops, including those eaten raw. It is assumed that in order to meet WHO guidelines, authorities would opt for a stabilisation pond treatment system with multiple ponds. The authorities would want to compare the cost and risks at that level of treatment with the cost and risks entailed if they did nothing and continued to irrigate vegetables with untreated wastewater, and alternately, if they adopted the USEPA/USAID recommended guidelines for treatment of vegetables eaten raw. For the purpose of this illustration only, the unit cost of wastewater treatment to meet the various guidelines can be roughly estimated as:

The estimate of treatment costs to meet WHO guidelines does not necessarily apply to all situations but is generally illustrative of a situation that may apply in hot sunny climates in developing countries where low-cost land is available for effective stabilisation pond treatment. The annual cost of treatment to the recommended WHO guidelines is estimated at some US$12,500,000 for a population of one million persons. According to this estimate, the additional annual cost for that city to meet the USEPA/USAID guidelines would be US$27,500,000.

Assuming that half the hypothetical city’s population of one million consumes wastewater-irrigated vegetables on a regular basis, and that the annual risk of contracting rotavirus, hepatitis A virus, V. cholera and S. typhi infections associated with the use of vegetables, eaten raw and irrigated with untreated wastewater is the worst case, it is assumed that these vegetable crops are currently irrigated with untreated wastewater, and based on conservative risk estimates some 20% of the exposed half of the population, or 100,000 people become ill every year from one of the four diseases.

There would be 10 (10 × 10^-5) cases of rotavirus, 5 (4.7 × 10^-6) cases of hepatatis A, and 23 (23 × 10^-5) cases each of cholera and typhoid, making 61 cases in all (Tables 5.1b and 5.3). If it is assumed that the USEPA/USAID guidelines, that call for no detectable FC/100 ml, entail an essentially zero risk of disease, then it can be estimated that these annual cases of diseases could have been prevented if the USEPA/ USAID microbial guidelines had been met. The additional cost of wastewater treatment would be about US$5,500,000 for each case of hepatitis A prevented. In the case of rotavirus disease, the cost would be some US$2,750,000; and US$1,200,000/case for V. cholera and S. typhi infection prevented. From Table 5.3 it can be seen also that: the greater the α value the higher the cost of prevention, that could reach as high as US$13.75 million to prevent a single case of hepatitis A. If it is assumed that all four infectious diseases are endemic and transmitted simultaneously then to prevent all 61 cases/ year resulting from the four listed pathogens, it would cost US$27,500,000, i.e. on average, the cost of preventing a single case would be US$451,000. Nevertheless, if the true level of risk associated with the WHO guidelines is closer to the 10^-6 level, then no detectable reduction of risk would be gained by the additional annual investment of US$27,500,000 required to meet the USEPA/USAID effluent guidelines. These figures are estimated by the less-conservative interpretation of the results of this study. It is questionable whether this level of additional treatment, requiring major extra expenditure, is justifiable to further reduce the negligible low levels of risk of infection and disease that these estimates indicate are associated with the new WHO guidelines.

Let us look at the cost-effectiveness of treating the wastewater to the WHO recommended guidelines for this city of one million as compared to the situation of continuing the irrigation of vegetables eaten raw with untreated wastewater. If the present state of no treatment and irrigation with untreated wastewater were to continue, the community would be faced with some 100,000 annual cases of the four enteric diseases included in this study. By building a treatment plant that achieves the WHO guidelines some 99,940 cases of disease could be prevented each year at an estimated annual total cost of some US$12,500,000 or US$125/case of disease prevented. This can be considered reasonably cost-effective and a worthwhile investment in public health disease prevention. However, each community must make its own judgment as to the level of investment it is prepared to make in preventing disease.

Table 5.3. The annual cost in a city with a population of one million of preventing a single case of a particular disease caused by a specific pathogen due to eating lettuce irrigated with effluent according to WHO guidelines (1000 FC/100 ml), at a rate of 100 g/day for a total of 150 days.

It should be recalled, however, that the health burden incurred by the different diseases varies, and that each disease should be considered separately. Accordingly, WHO and the World Bank have developed another method of evaluating health risk by comparing different diseases on one scale, disability adjusted life years (DALY) (Murray and Lopez, 1996).

Disability Adjusted Life Years (DALYs)

In this study the health effects of the four infectious diseases are considered equally, the WHO and the World Bank have developed a new methodology that measures their relative public health burden by comparing the weight of the damage incurred by the diseases (DALYs) rather than by counting the total number of cases of each disease. DALY emphasises the real health weight of the diseases, that might in some cases be fatal and/ or cause long-term damage such as liver injury due to hepatitis A or paralysis in poliomyelitis. This integrated measure combines the number of years of life lost (YLL) by mortality with the number of years lived with a disability (YLD). These are standardised by severity weights. DALY is equal to the sum of YLL + YLD. YLL is calculated by multiplying age-specific mortality rates by the life expectancy of the fatal cases that have not developed the disease. YLD is calculated by multiplying the number of cases by the average duration of the disease and a weight factor that reflects the severity of the disease on a scale of 0–1 (death).

As an example, the DALY of two intestinal diseases: hepatitis A and salmonellosis is calculated:

DALY for 1000 cases of hepatitis A

DALY for 1000 cases of salmonellosis

It can be seen that in this example the disease that has real public health burden is hepatitis A and not salmonellosis (the weight of damage of one case of hepatitis A is equal to 1,378 cases of salmonellosis), since hepatitis A causes death or has a life-long effect. Therefore, an approach that considers the number of cases rather than the weight of diseases according to their real damage (calculated in DALY) is less accurate. It is more justifiable to calculate cost-effectiveness based on preventing diseases like hepatitis A or poliomyelitis that cause heavy health damage, rather than salmonellosis or rotavirus infections. The use of the DALY approach is more logical for this type of risk/ cost-effectiveness analysis. For example, it might be more reasonable just to estimate the cost of preventing the one important disease (hepatitis A) rather than pooling all the other less-important infectious diseases (Shuval et al., 1997).

Discussion and Conclusions

A model for the assessment of risk of infection and disease associated with wastewater irrigation of vegetables, eaten raw, has been developed based on a modification of the Haas et al. (1993) risk-assessment model for drinking water. The modifications include laboratory experiments to determine the amount of wastewater that could cling to such irrigated vegetables as cucumbers and lettuce, and an estimation of the concentration of pathogens that would be ingested by consuming vegetables irrigated with wastewater of different standards. Validation of the model with data from the Jerusalem cholera epidemic and typhoid fever outbreaks in Santiago which, in both cases, were caused primarily by the consumption of wastewater-irrigated vegetables, lends support to the assumption that the risk-assessment model can provide a reasonable approximation of the levels of disease that really can and have occurred due to irrigation with poor-quality wastewater. Risk assessment, using this model of irrigation with treated wastewater effluent that meets the WHO guidelines for vegetables eaten raw (1000 FC/100 ml), indicates that the annual primary infection risk of a disease such as hepatitis A is about 10^-5 to 10^-6, and of diseases caused by rotavirus, V. cholera, and S. typhi – about 10^-5 to10^-6.

It is worth mentioning that in developing the risk-assessment model, the worst possible scenario was used in order to reduce the uncertainty factor, and that disease transmission due to secondary infection was not taken into consideration. Therefore, the total number of cases may be higher than the number estimated on the basis of primary infection. The USEPA has determined that guidelines for drinking water microbial standards should be designed to ensure that human populations are not subjected to an annual risk of enteric disease infection greater than 10^-4 (Regli et al., 1991). Thus, this study suggests that the WHO wastewater effluent reuse guidelines provide a safety factor some one to two orders of magnitude greater than that called for by the USEPA for microbial standards for drinking water. Current findings correlated well with those recommended by Blumenthal et al. (2000), based on the revised WHO guidelines for treated wastewater used for agriculture (WHO, 1989).

According to the cost-effective analysis, the data suggest that the additional degree of risk reduction that might be attained by meeting the USEPA/USAID guidelines for water reuse (that require no detectable FC/100 ml), would, according to the most conservative estimate, result in expenditure of some US$1.2–5.5 million per case of disease prevented when α = 0.2. However, if α = 0.5 the cost would be as high as US$13.75 million. It is questionable whether such additional investments in high technology wastewater treatment facilities designed to meet the USEPA/USAID guidelines rather than the WHO guidelines, are justifiable, considering the small degree of additional health protection they might provide. However, the variable health burden incurred by the different diseases calculated as DALYs should also be considered.

Major chapters in this volume are devoted to the views of their authors on the benefits of using untreated wastewater in agriculture. In these authors’ estimates the risk of becoming ill with an infectious disease, including very serious diseases with significant death rates and long-term consequences such as hepatitis A, from the consumption of salad crops irrigated with untreated wastewater is very high. It is conservatively estimated that some 20% of the exposed population (those eating raw vegetables) will become ill every year with one of the four diseases included in this study if they eat vegetables irrigated with untreated wastewater. The cost-effectiveness of treating wastewater to the WHO recommended guidelines against continuing to irrigate vegetables eaten raw with untreated wastewater would be about US$125/case of disease prevented. This can be considered a reasonably cost-effective level and a worthwhile investment in public health disease prevention. However, each community must make its own judgment on the level of investment it is prepared to make in preventing disease.

It must be pointed out that the model used in this study estimates the risk of infection and disease only of those who consume raw vegetables irrigated with untreated wastewater. It does not include the health risks to the farmers and irrigation workers exposed to untreated wastewater. Earlier studies (Shuval et al., 1986) have shown that these risks are considerable, particularly in areas where hookworm and other parasitic diseases are endemic. Thus, in the authors’ view, irrigating vegetable crops eaten raw with untreated wastewater is not a desirable public health practice. Treating wastewater to significantly reduce the concentration of pathogens along the lines recommended by the WHO appears to be the right way to go. But even somewhat less-rigorous treatment levels that are less costly could provide significant cost-effective health benefits. This study did not evaluate such alternative degrees of treatment.

It should also be noted that one of the common risks associated with present lifestyles is road accidents, which in Israel alone, reach an annual total of 7 × 10³ injured. This value is similar to the risk of infection from eating untreated wastewater-irrigated vegetables, which can be lowered by 2–3 orders of magnitude if wastewater is treated to meet the WHO guidelines. Then too, injuries incurred by road accidents are far more serious and lethal than the enteric diseases resulting from the ingestion of vegetables irrigated with wastewater effluent. This example is presented in order to raise the issue that health-protecting investment should bear some rational relationship to the risks involved and the cost-effectiveness of the preventive measures.

Acknowledgements

This project was funded by a grant from the US Agency for International Development (USAID) and the Middle East Regional Co-operation (MERC). We are grateful to Dr Noya Galai of the Hebrew University, Hadassah School of Public Health for her advice on the biostatistical analyses, and to Dr Nava Haruvi of Volcani Institute, Israel, for her advice on economic aspects of wastewater use. The editorial suggestions of Dr Chris Scott are much appreciated.

References

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Document(s) 7 of 19