I agree with Fullerton and Semple that indoor air pollution is another major environmental concern—in addition to outdoor air pollution—that affects both human health and climate change. But calling indoor air pollution the “real key to reducing the burden of ill health” carries their point too far. While these are both important factors, there are also many other causes of ill health throughout the world besides indoor and outdoor air pollution, including malnutrition, obesity, sexual and reproductive health risks, tobacco use, unsafe water, and the list goes on from there.1

In my editorial I focused especially on outdoor air pollution, not because indoor air pollution and the other causes of global ill health are less important, but because outdoor air pollution was the topic of the Thorax journal article by Ko et al2 about which I was writing.

However, since the issue has been raised, it is important for the reader to note that the health impact numbers cited by Fullerton and Semple for indoor air pollution are highly uncertain. Indeed, a recent review concluded that one of the key research questions in this field is: “What is the quantitative relationship between exposure to indoor air pollution and the incidence of disease (ie, the exposure–response relationship)?”.3 Even the reference that Fullerton and Semple rely upon4 acknowledges that: “It is questionable, however, whether exposure–response relationships derived from pollutant-based investigations are applicable to populations exposed to indoor air pollution in rural areas of developing countries, since most pollutant-based epidemiological studies were conducted outdoors in urban areas of developed countries. Potential problems include differences in pollutant mix and composition, exposure patterns and levels, and population characteristics. The chemical pollutants produced by burning solid fuels, for example, are different from those produced by burning fossil fuels.” In fact, to address just this need for better estimates for use by policy makers, there is a commendable new major effort to improve and update such estimates being initiated (http://www.globalburden.org). Thus, given the large uncertainties surrounding present such global estimates, using them to rank the importance of various causes of ill health seems inappropriate at this time.

In conclusion, I would agree with Fullerton and Semple that indoor air pollution is also a concern regarding both human health effects and climate change, but one that needs—and deserves—much more extensive direct research regarding the size of its potential for human health effects as a function of both fuel type and population.