Per Annette's excellent suggestion, this is a discussion thread where interested readers can participate in a discussion of the scientific evidence regarding the health effects of secondhand smoke. I will initiate discussion by presenting some of the reasoning that I and other researchers have used in concluding that secondhand smoke is the cause of various diseases. I will then make myself available to try to answer questions readers have in understanding the reasoning behind those conclusions or in responding to arguments against the conclusions.
However, I want to emphasize that I do not view this as an attempt to convince anyone of anything. Instead, it is intended more as a forum where people can discuss the issues, ask questions, and more importantly, hopefully come to a better understanding of why different people have come to various conclusions. Hopefully, by understanding where people are coming from, it will help to break down some of the stereotypes (largely derived from certain anti-smoking groups) that those who disagree with certain conclusions are merely tobacco industry fronts.
The first topic for discussion will be the California Environmental Protection Agency's conclusion that secondhand smoke is a cause of heart disease in nonsmokers. This conclusion, and the basis for it, is discussed in Chapter 8 of the Cal-EPA report.
A link to this discussion thread has been placed in the upper right-hand corner of my blog, so that you can come back and contribute to and/or read the discussion thread anytime, and so that we can move on to other topics as we go forward.
I will initiate the discussion by bringing up an important aspect of the process by which epidemiologists evaluate a causal hypothesis. Perhaps the first step is to determine whether there is a consistent relationship between the exposure and the disease in question and whether that association, if present, could simply be a result of chance.
The process by which the results of the studies are evaluated is called meta-analysis. This is an attempt to pool together all the relevant studies on a topic and to see if viewed all together, there is a consistent relationship between the exposure and the disease. Here, we are looking at studies that have evaluated the risk of heart disease in relation to exposure to secondhand smoke. The Cal-EPA report reviewed 24 studies.
Three different meta-analyses have been conducted of these studies. One meta-analysis, which included 18 studies, found that people exposed to secondhand smoke were about 1.2 times more likely to develop heart disease (this is called a "relative risk" of 1.2). Another meta-analysis, of 8 studies, found a relative risk of 1.3. And a third meta-analysis, of 19 studies, found a relative risk of 1.2. All of these relative risks were statistically significant, meaning that they could not likely be a result of chance.
From this analysis of all the studies taken together, the Cal-EPA concluded that there is an association betweeen secondhand smoke exposure and increased risk of heart disease, albeit a small one (the relative risk of 1.2-1.3 indicates that exposed nonsmokers are about 20-30% more likely to develop heart disease than non-exposed nonsmokers), and that this association cannot be explained by chance. It is not just a chance finding that the studies happened to find that secondhand smoke exposure is associated with increased risk of heart disease.
This is the point I'd like to stop at for now. It is important for people to understand this initial aspect of the analysis before getting into a discussion of the possible reasons for this observed association between secondhand smoke exposure and heart disease.