Indoor air pollution

Smoke and malaria: are interventions to reduce exposure to indoor air pollution likely to increase exposure to mosquitoes and malaria?

Half of the world’s population relies on biomass or coal as their primary household fuel source. The smoke produced from burning these fuels indoors presents a major health risk. Much work is being done to develop interventions to reduce levels of indoor air pollution. However, anecdotal evidence exists for smoke having a repellent effect on mosquitoes. This raises the possibility that interventions to reduce indoor air pollution might increase the risk of malaria and other insect-borne diseases. Eighty percent of the burden of insect-borne diseases and ninety percent of the resulting deaths are due to malaria and the report therefore focuses on this disease. A literature review was performed to assess the extent of evidence for smoke providing protection from malaria or mosquitoes. Although there is evidence that the smoke from certain plant products contains active compounds that have a repellent effect, no experimental evidence was found for a repellent effect attributable to smoke from domestic biomass fuels.

One of the important findings from this study is the dearth of literature relating to the effects of smoke from domestic fuel use on mosquito behaviour. The conclusions and tentative recommendations presented with respect to this are therefore grounded in a very weak evidence base. Furthermore, vector behaviour and malaria epidemiology vary across geographical settings making it unlikely that a single rule describing the relationship between smoke, vectors and the incidence of vector borne disease would have global applicability. Keeping this in mind, the main findings of the study with its focus on Africa are as follows:

  • Smoke from domestic fuel use probably does not have much effect on mosquito feeding, but mosquito feeding is affected by smoke from certain plant products traditionally used as repellents.
  • The primary biting time for the world’s most important malaria vectors is late at night when cooking activities are likely to have ceased.
  • The presence of eaves spaces makes houses more permeable to mosquitoes and has been associated with an increased risk of malaria in observational studies in some contexts. Eaves spaces, as well as doors and windows, can be protected to some extent against mosquito entry by using screens or curtains. The protection is increased if these are treated with insecticide. However, the extent to which such measures compromise ventilation is not known.

On this basis there seems to be a good health argument for continuing efforts to reduce indoor air pollution, even in areas where malaria is endemic. It is extremely likely that such efforts will have substantial health benefits in reducing respiratory disease, and extremely unlikely that the reduction of smoke per se will have any significant health costs in terms of increased malaria. The publication concludes with a summary of research needs and public health recommendations.

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