A downward spiral: the hazards of mosquito coils.

In many tropical and subtropical countries, burning mosquito coils is a key strategy for reducing mosquito bites. But while mosquito bites can be particularly dangerous in these areas due to endemic mosquito-borne diseases such as malaria, the prophylaxis may pose its own hazards. In this issue, Weili Liu of the University of Medicine and Dentistry of New Jersey and colleagues present a laboratory study of the indoor air pollution created by six popular brands of mosquito coils from China and Malaysia [EHP 111:1454-1460].

Coils typically contain 0.3-0.4% pyrethrin insecticides by weight, with the balance being combustible materials chosen for their ability to smolder without flame. Although the plant-derived pyrethrins are relatively nontoxic to humans, the health effects of burning the binders, fillers, and dyes composing the other 99%-plus of the coils are uncertain, especially considering the intensive, long-term exposure: coils are typically burned all night for several months or even all year (depending on mosquito conditions) in rooms where ventilation is often limited to increase the coils' effectiveness.

The researchers burned the test coils in a 0.15-cubic-meter chamber and a 32-cubic-meter room, and periodically withdrew air samples for analysis of concentrations of particulate matter, polycyclic aromatic hydrocarbons, aldehydes, and volatile organic compounds.

The particulate concentrations peaked in two size ranges, 0.01-0.1 micron and 0.2-0.3 micron. These ultrafine and fine particles lodge deep in the lung, and in other studies have been implicated in lung disease and premature deaths from all causes. The researchers calculated that 1 coil released as much particulate matter 2.5 microns in diameter or smaller (P[M.sub.2.5]) as would be released from 75-137 cigarettes. In terms of estimated ambient room concentrations, the researchers made a "conservative estimate" that P[M.sub.2.5] was six times higher than the U.S. National Ambient Air Quality Standard for 24-hour exposure.

The smoke contained three polycyclic aromatic hydrocarbons classified by the U.S. Environmental Protection Agency as probable human carcinogens--benzo[a]pyrene, benzo[b]fluoranthene, and benzo[k]fluoranthene. The researchers also found relatively high concentrations of volatile organic compounds such as benzene, a neurotoxicant and carcinogen that can affect bone marrow with chronic exposure.

In addition to the possible long-term damage that many of the mosquito coil pollutants may cause, the researchers concluded that significant acute health effects, including asthmatic reactions and eye and respiratory irritation, could be expected from exposure to the combustion product acrolein.

The incomplete combustion seen in mosquito coils also occurs when biomass fuels such as cow dung, crop residue, and wood are burned. In other studies, use of these fuels has been correlated with respiratory diseases including chronic obstructive pulmonary disease. However, in making their analysis, the authors did not try to evaluate the overall health impact of mosquito coils--they did not, in other words, balance the benefits of reduced exposure to malaria and other mosquito-borne diseases against the smoke-related impacts of the coils.

They did note, however, that the Chinese coils emitted significantly less particulate matter, perhaps due to the material used as a smoldering agent (the Chinese products were thought to contain more sawdust, whereas the Malaysian coils were based on coconut husks and shells). The researchers suggest that the health impact of mosquito coils could be reduced, regardless of whether smokeless mosquito controls (such as window screens) are adopted, by switching to a less-polluting formulation.