A very large risk indeed

A coal fired stove in China (photo from here)

In yesterday's post, I wrote about ultra-low epidemiological results and their effect on absolute risks. In a nutshell, the effect is to make low absolute risks ever-so-slightly higher, but not high enough to make any rational person radically alter their lifestyle.

Relative risks of 1.01 to 1.50 (ie. an increase of risk of 1%-50%) are very common in published epidemiological studies and have been for a couple of decades. This is natural enough, since there are more and more eager epidemiologists chasing fewer and fewer 'epidemics'. The problem is that findings of this order are so low that they can easily be produced by any flaw or bias in the study design, not to mention sheer chance. (See Gary Taubes' classic article Epidemiology Faces Its Limits.)

It might be true that light drinking increases a woman's breast cancer risk by 5%, for example, but a finding of 1.05 is so close to a finding of 1.00 (ie. zero increased risk) that it might readily be explained by confounding by known or unknown risk factors. Even if the risk is real, it changes a woman's lifetime breast cancer risk by such a small amount that it has little clinical significance.

Occasionally, however, a study comes along which reminds us what meaningful risk really is. This paper from the British Medical Journal deserved more media attention than it received, which was none at all. It looked at  the risk of lung cancer from the use of traditional coal-fuelled cooking methods in China and found...

After adjustment for the presence of competing risks, the probability of death due to lung cancer before 70 years of age for men and women using smoky coal was 18% and 20%, respectively, whereas it was less than 0.5% among individuals of both sexes using smokeless coal.

To be clear, that is not an 18%-20% increase in risk, but an 18%-20% absolute risk. One in five users of smoky coal will die of lung cancer, compared to one in 200 smokeless coal users.

Looking at the relative risks, we are greeted with extraordinarily big numbers and a sharp contrast between the genders which strongly suggests that the correlations are causal (the authors note that "only a small proportion of men in the cohort cooked").

...lifelong use of smoky coal compared with smokeless coal was associated with a 36-fold increase in lung cancer mortality in men and a 99-fold increase in women.

To put those risks in the terms usually used by the media, that's a 3,600% increase in risk for men and a 9,900% increase in risk for women.

Lung cancer alone accounted for about 40% of deaths before age 60 among individuals using smoky coal

A shocking and remarkable statistic. Of course, we have long known that smoke of any kind is bad for the lungs and since tobacco is such a large source of smoke in the West, it is unsurprising that it gets so much attention, but it is worth remembering that the rest of the world still has major problems with industrial and domestic pollution. The authors note that a 1993 survey found that 70% of China's rural population use coal for cooking and such fuels are used by 3 billion people worldwide. Considering the sketchy epidemiological guff that makes it into the media on a daily basis, studies like this warrant at least a passing mention.