Arsenic, drinking-water and health risk substitution in arsenic mitigation: A discussion paper
Nature of hazards that may substitute for arsenic
There are three principal types of hazard that could be expected to potentially substitute for arsenic from water supply provided during an emergency response. These are:
- Microbial hazards: pathogens derived from human and animal faeces that cause diarrhoea, as well as a range of other diseases, some with significant chronic sequelae;
- Toxins derived from cyanobacteria that may lead to adverse health effects including liver cancer; and,
- Chemical contaminants in source water introduced from pollution.
Comparing risks from microbial hazards and arsenic
Microbial hazards represent an overall greater threat than chemical hazards and in developing countries account for a significant proportion of the burden of disease. Diseases due to microbial hazards from poor water, sanitation and hygiene are responsible for 5.7% of the total global burden of disease. For microbial hazards, as for carcinogenic risk from arsenic, it is assumed that no safe threshold exists and that any exposure has the potential to initiate an adverse health effect.
When comparing the risks associated with arsenic and microbial hazards, several important points emerge. Both are strongly influenced by poverty and nutrition. Risks of infection by microbial hazards (pathogens) increases markedly with increasing poverty. The overall health burden from pathogens is significantly greater in poorer communities. Arsenicosis also appears to be related to poverty and has a greater incidence among poorer households exposed to elevated concentrations of arsenic. For both pathogens and arsenic, poor nutrition is likely to contribute to greater susceptibility. In the case of microbial hazards, repeated infection also significantly contributes to under-nutrition.
The degree of uncertainty regarding the epidemiology of arsenic-related health effects and the progression of arsenicosis makes quantitative risk comparisons with health effects from microbial hazards difficult. Equally, there is significant uncertainty regarding the role of drinking-water in infectious disease transmission in Bangladesh, primarily because of the limited water quality data and the limitations of data solely expressed in term of index organisms. Undertaking quantitative risk assessment is certainly possible, but would require collection of further data on target pathogens in drinking water and this should be considered as a priority in the short-term. However, although quantitative risk comparison may be difficult, qualitative comparisons are possible and are outlined below.
Nature of health effects
The nature of health effects between microbial hazards and arsenic are very different. Arsenicosis is essentially a chronic disease and there is a significant latency period before symptoms are developed. There appears to be discrepancy in the literature regarding latency, with some reports of 2 years being the minimum for hyperpigmentation and keratosis. Researchers in Bangladesh suggest that 5 years is the minimum latency, whilst some other estimates suggest that this is 9 years. Latency for cancers is also unknown, but it is estimated to be of the order of 20 years.
Microbial hazards typically lead to acute health effects with (in all but a few cases) incubation periods of typically hours to days. Most episodes of infection lead to self-limiting diarrhoea provided fluid replacement is practised. However, all pathogens can lead to mortality and this may be significant in sensitive sub-populations, notably infants and children (all pathogens), immune compromised (often to specific opportunistic pathogens) and pregnant women (specifically in relation to hepatitis E virus). Furthermore, although many episodes of diarrhoea are in themselves self-limiting, there is a synergistic relationship with under-nutrition from repeated episodes.
The proportion of a population exposed to elevated arsenic from drinking-water that will go on to develop arsenicosis is unknown. WHO have modelled the progression of arsenicosis using data from Samta, Bangladesh. The range of those affected over 30 years was 15.75% in the lowest estimate scenario to 29.25% in the highest estimate scenario. Variation in the estimates of mortality from cancers was between 5.0 and 6.5%. This implies a significant overall health burden for those affected.
Estimating the number of people that will develop symptoms of an infectious disease from exposure to pathogens is also uncertain, as this depends in part on the dose ingested and susceptibility of the host, but outbreak data suggest that this is in the range of 20% to 70%. The proportion of those who become infected who die varies, but is typically low among healthy adults with much higher rates for sensitive sub-populations.
Medical treatment for microbial hazards is generally well-understood, although in practice access to the required interventions may be limited, particularly among the poor. Some sequels (e.g. reactive arthritis) may be more problematic to treat. Treatment of arsenicosis remains an area of uncertainty. Current evidence suggests that during early onset, switching to arsenic safe water reverses symptoms, although there is a lack of controlled trials in Bangladesh on which to validate this and to identify the stage at which this is no longer effective. It is not clear whether early removal of arsenic- contaminated water would reduce the onset of cancers, but it is assumed that it would have some impact because of the cumulative nature of the risk.
More recent work suggests that anti-oxidants within vitamins A, C and E and possibly compounds containing zinc and selenium also work to reverse symptoms. A recent controlled trial was performed in Bangladesh, but this remains to be published and may require further controlled clinical trials. However, this does indicate the necessity of combining both environmental and medical interventions for arsenicosis.
The nature of the acute health effect from microbial hazards, the particular impact on sensitive sub-populations, the typical attack rates and the synergistic relationship with under-nutrition show that the risk posed by microbial hazards is greater than for arsenic. This does not imply that arsenic mitigation is not important, but to emphasise the need for emergency response measures to ensure that risk from microbial hazards do not increase.
Experience shows that control of microbial hazards in the technologies considered for the emergency response is possible, but that in order to achieve this control actions are required in the short and long-term. The Section 3 of this paper will identify specific issues that need to be considered and suggest ways in which control can be maintained. Examples of water safety plans for most of the technologies are provided in the supporting documents. It should also be recognised that if technologies are introduced that are not acceptable to the end users then not only will the risk from arsenic continue to threaten the health of some of the population, but risks from microbial hazard may also increase as water supplies deteriorate. Part of the acceptability will include the typically greater distance to the source that will result from most of the options considered in the emergency response. There will be a need for ongoing education and effective risk communication to prevent households from maintaining use of existing contaminated tubewells for water for drinking and cooking.
Comparing risks from other potential hazards
The risks associated with toxins derived from cyanobacteria include liver cancer. Other effects include acute poisoning from immersion in water where there is a bloom. Cyanobacterial blooms are found in surface waters with high nutrient loads and recent work in Bangladesh has identified that these blooms occur in some ponds in the country. Nutrient loads may be derived from general pollution and commercial fish-farming may further contribute to phosphate and nitrate input. In addition to direct adverse health effects, algal blooms often lead to significant taste and odour problems that may lead to the rejection of a source by users. Although toxins from microcystis (particularly microcystin-LR) may lead to cancer end-points, the overall health impact of cyanobacterial toxins remains unclear. Within the short-time horizon of an emergency response it is unlikely to represent a greater risk than drinking arsenic contaminated water. Furthermore, as discussed below some removal of toxins may be possible through water treatment. In the longer-term response to arsenic, however, the risks from cyanobacterial toxins may be more significant given the extended duration of consumption. This will therefore need to be addressed in defining water supply options. It would seem unwise at this stage to consider the use of a water source that is known to be affected by cyanobacterial blooms for any intervention likely to extend beyond a very short-time period and only then if no other options were available and there was strong preference for communities for use of pond water. Other potential hazards include chemical derived from pollution, for instance nitrate and pesticides from agriculture and heavy metals from industry and air pollution. Although there is evidence of pollution of surface waters and from air pollution in urban areas, the overall risks associated with such pollution will be significantly lower than for microbial hazards and arsenic.