## **Box  3.2  Actual and potential health risks in wastewater irrigation**

An _actual_ risk to public health occurs as a result of wastewater irrigation when all of the four following conditions are satisfied:

- (1) either an infective dose of the pathogen reaches the wastewater-irrigated field or the pathogen multiples in the field to form an infective dose

- (2) the infective dose reaches a human host

- (3) the host becomes infected

- (4) the infection causes disease or further transmission.

Actual risks can thus only be determined from epidemiological studies.

If conditions 1−3 are satisfied but not condition 4, then the risk is only a _potential_ risk.

_Source:_ WHO, 1989.

There was less compelling evidence for the transmission of viral and protozoan diseases. Blumenthal and Peasey (2002) reviewed the epidemiological evidence reported after the study by Shuval _et al_ . (1986). The main findings of their study were:

- _Unrestricted irrigation_ : The use of untreated wastewater to irrigate vegetables led to increased helminth infection (mainly _Ascaris lumbricoides_ infection), bacterial infections (typhoid, cholera, _Helicobacter pylori_ infection), and symptomatic diarrheal disease in consumers. When wastewater was partially treated, there was evidence that the risk of bacterial and viral enteric infections was still significant when consumers ate some types of uncooked vegetables irrigated by water containing ≥10<sup>5</sup> fecal coliforms per 100 mL (see Box 3.3).

- _Restricted irrigation_ : Studies of the risks of viral and bacterial enteric infections related to use of treated wastewater suggested that when sprinkler irrigation was used and the population was exposed to wastewater aerosols, there was an increased risk of infection when the quality of the wastewater was 10<sup>6</sup> total coliforms per 100 mL, but no increased risk of infection when the quality of the wastewater was 10<sup>3</sup> −10<sup>4</sup> fecal coliforms per 100 mL. Studies of the risks of symptomatic diarrheal disease and enteric viral infections related to direct contact with treated wastewater through farm work (adults and children) or play suggested that, when flood or furrow irrigation occurs, there was an increased risk of infection in children when the quality of the wastewater was >10<sup>4</sup> fecal coliforms per 100 mL. For adults, the threshold level for symptomatic diarrheal disease was 10<sup>5</sup> fecal coliforms per 100 mL, but the threshold level for transmission of a Mexican strain of norovirus was <10<sup>4</sup> fecal coliforms per 100 mL where high levels of contact occurred, even in a rural area where there were many other transmission routes for this virus.

# _Significance of microbial health risks_

Feachem _et al_ . (1983) considered that the health problems associated with wastewater use in the production of human food could be broken down into a series of questions:

- How many pathogens and of what kind reach the field or crop?

- Are pathogens likely to survive in sufficient numbers and for sufficient time to cause subsequent infection?

- How significant is this infection route compared to all other potential infection routes?

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