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Secondhand Smoke and Lung Cancer

About 160,000 people die of lung cancer each year. About 85 to 90% of that cancer can be attributed to active smoking. The other 10-15% are caused by factors other than active smoking.

If we took lung cancer in never smokers, the term for people who never smoked, it would represent 16 to 24 thousand deaths per year, a major cause of cancer death. It's not something we should overlook.

Why do never smokers develop lung cancer? Well, for a sizable proportion we aren't quite sure what the risk factors were, but for at least part of the cases, we can positively link it to known and suspected carcinogens in the air. Particularly radon, second hand smoke and indoor air pollutants. There are also differences in the rates between men and women, with women never smokers slightly more likely to develop lung cancer, although that may be simply a matter of the relatively higher longevity of women.

What I'm really interested in, in this short video, is the risk increase from second-hand smoke. It's a topic that's been of interest since we first realized that smoking was causing major increases in lung cancer. A series of major studies have only very recently verified that second hand smoke in the home, in the workplace and in public places directly contributes to lung cancer risk in never smokers. I'm going to present just some very basic results from two of these reviews, one published in Clinical Cancer Research in September of 2011 and the other in the International Journal of Cancer published in 2004.

Both of these papers are available as free full-text articles in links below:

Int J Cancer. 2004 Mar;109(1):125-31.
Secondhand smoke exposure in adulthood and risk of lung cancer among never smokers: a pooled analysis of two large studies.

Clin Cancer Res. 2009 Sep 15;15(18):5626-45.
Lung cancer in never smokers: clinical epidemiology and environmental risk factors.

The CCR2011 review gives a lot of background on lung cancer in never smokers, and it goes briefly into what is known about the plausibility or mechanism by which we think second hand smoke causes cancer in never smokers. Suppose Dad smokes and Mom doesn't.

If we test Mom's blood for tobacco specific compounds that are known carcinogens, we can find that they are elevated even after she leaves the home environment for several hours. We know that there are differences between the mainstream smoke that Dad inhales through a filter and the sidestream smoke that Mom is exposed to from a burning or smoldering cigarette.

On average, this sidestream smoke is about four times as toxic per gram. It's toxic when inhaled, toxic on the skin, and much more irritating to mucosa in your eyes, lungs and throat than mainstream smoke (Tob Control. 2005 December; 14(6): 396–404.).

So it comes as no surprise that the never smoker wives of smoking husbands have much higher rates of various types of cancer. The increase is somewhere around 30%, depending on the population and the amount of exposure. These never smoker wives also, by similar mechanisms, have higher rates of heart disease.

These effects are dose dependent almost right down to the very lowest exposure. That is, unlike a lot of other carcinogens, there is no known safe level of exposure to second hand smoke that causes no extra health risk.

It's an additive effect. Before I go any further I need to clear up one possible misconception. Active smoking carries a much higher risk of lung cancer than passive smoking or second hand smoke. An active smoker might be 10 to 20 times more likely to develop lung cancer than a never smoker, depending on a lot of factors. The 30% lung cancer increase is relatively small by comparison, but still represents a major health risk.

Going back to the paper, data from the National Human Activity Pattern Survey showed that in exposed never-smokers as a group, that 43% of that exposure is in the home, 7% in the workplace, 9% in a vehicle, and 15% in a bar or restaurant.

The second paper found that, in two very large case-control studies pooled together between the US and European populations, long-term exposure to workplace second hand smoke increased lung cancer risk by 25% where long-term exposure at home increased risk by 23%. Social exposure in bars, restaurants and in cars likewise increased risks by about 26%. This may be an underestimate. Other, similar reviews of large populations found between 25 and 45% increases.

When we look at the comprehensive US statistics, the range of people who die from lung cancer as a result of someone else smoking is probably between 3000 and 5000 people per year as of 2003. If we express that as a fraction of the total number of lung cancer deaths in never smokers, we come up with a number between 20 and 50% of all lung cancer deaths in never smokers being attributable to second hand smoke.

If you are a never smoker who lives or works with second hand smoke, bear in mind that has an impact on your health. Assert yourself, protect your lungs, and good health to you. Credited: C0nc0rdance

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