# **2. Previous research on the effects of weather on health outcomes**

A model of potential pathways through which climate change impacts health is constructed by McMichael et al. (2006). In this model extreme weather events are one of the manifestations of environmental change. The impacts of extreme weather events on health are seen through deaths and illness from thermal stress; microbial proliferation which can result in unsafe water and food poisoning; reduced crop, livestock and fishery yields which lead to impaired nutrition, health and survival and loss of livelihoods.

Although studies have already investigated the impacts of extreme weather events on a range of health outcomes, relatively few have quantified weather effects at high resolution. IPCC (2007) details several studies that have documented the incidence of diarrhea in the aftermath and during floods, for example India (Mondal et al., 2001), Bangladesh (Kunnii et al., 2002; Schwartz et al., 2006) and Mozambique (Cairncross and Alvarinho, 2006). Moreover O’Shea and Field (1992) illustrate the connections between the functioning of infrastructure and extreme weather events: floods may cause contamination of water sources as sewerage systems fail to function with heavy water loads, and storm run-offs can cause proliferation of pathogens. These papers concentrate on the links between water levels and disease, rather than directly on rainfall levels, although the two are, of course, related.

The direct links between excessive rainfall and the incidence of waterborne disease outbreaks are established by Curreiro et al. (2001) by analyzing a database of all reported waterborne disease outbreaks in the USA from 1948 to 1994. Results show that 51 percent of waterborne disease outbreaks were preceded by precipitation events above the 80<sup>th</sup> percentile of monthly average for the particular weather station. The time period for the disease outbreak to occur varied: those due to surface water contamination showed the strongest association with extreme precipitation in the month of the outbreak whereas for groundwater contamination a lag of up to two months was allowed. Surface water is quickly contaminated during heavy rainfall events, but the period in which groundwater contamination can occur may be longer. The difficulties of establishing the lag within which rainfall can impact diarrheal incidence led Hashizume et al. (2007) to analyze the impact of rainfall above a threshold on the incidence of non-cholera diarrhea over a period of 16 weeks. The possible ecological pathways through which climatic changes, including higher precipitation, impact in relation to the specific pathogen vibrio cholera are discussed by Colwell (1996). Although there is strong evidence that extremely high precipitation leads to an increased incidence of diarrhea, it should also be noted that higher rainfall levels could potentially improve water quality through the dilution of pathogens (Hunter et al., 2002).

Extreme high or low rainfall events have also been found to impact nutritional status. Hoddinott and Kinsey (2001) find that in Zimbabwe children aged 12 to 24 months lose 1.5cm to 2.0cm of growth in the aftermath of a drought. In this study, they calculate the percentage of rainfall in a year as a percentage of the long-term average over a period in excess of 20 years and match children who were initially aged 12 to 24 months to drought conditions, as children in this age group are most likely to suffer growth suppression 5