Accompanied by a press release and news conference, the Surgeon General of the US has released an extensive report (as PDFs, it runs well over 10 MB) on the current state of knowledge about the health risks of second hand smoke. Given the process that generated the report, it's no surprise that it's pretty hefty:
The Report was written by 22 national experts who were selected as primary authors. The Report chapters were reviewed by 40 peer reviewers, and the entire Report was reviewed by 30 independent scientists and by lead scientists within the Centers for Disease Control and Prevention and the Department of Health and Human Services.
A general feel for the content can be had by reading the major conclusions of the report. Some of the highlights are that nearly half of non-smokers in the US have detectable tobacco smoke contents in their blood, and that many of the compounds in second hand smoke cause immediate changes in the physiology of the lungs and cardiovascular systems, even at very low concentrations. One of the more striking conclusions was that the only way to avoid workplace exposure was to go completely smoke free. Conventional air cleaning systems don't really help remove the smaller particles and gasses in smoke, and will typically help distribute the air from designated smoking areas throughout the building.
A surprise to me was the association of second hand smoke with sudden infant death syndrome (SIDS), so I read through that chapter of the report more carefully. This section of the report is clearly the result of a large compendium of research from a variety of sources, and it includes both citations and summaries of the data used. The authors aren't afraid to point out where evidence for risk is lacking, such as in the case of miscarriages: "The evidence is inadequate to infer the presence or absence of a causal relationship between maternal exposure to secondhand smoke during pregnancy and spontaneous abortion." Which makes the conclusion on SIDS that much more startling: "The epidemiologic evidence for secondhand smoke exposure from postpartum maternal smoking associated with the risk of SIDS is consistent and strong…"
Scanning through some of the other chapters, it's clear that equal care was devoted to the rest of the report. The thoroughness and caution displayed in many sections of the report, however, may be undermined by the phrasing of one of the major conclusions, the one I expect is most likely to wind up as a sound bite in news reports on this topic: "The scientific evidence indicates that there is no risk-free level of exposure to secondhand smoke." Clearly, there must be some exposure level that is so fleeting as to not convey risk. And levels of risk will presumably depend entirely on which aspect is being examined. The risk of heart problems will undoubtedly register at a different level of exposure for than that for the risk of SIDS. A broad generalization like this may make it easier for those who seek to undermine the conclusions to attack the report as a whole.
Ultimately, regardless of its merits and flaws, the significance of this report will be measured in terms of how it affects policy. Advice from the surgeon general can result in anything from legislation (see your nearest cigarette package) to the resignation of the person providing the advice (see Jocelyn Elders). The current administration tends to be regulation-adverse, but many state and local governments show no such hesitation, so I expect that the largest impact of this federal report will be on local regulations.