National Post (National Edition)
Breathe — it won’t kill you
With summer here and people wanting to spend more time outdoors, we are harassed with cautions about air pollution, including the claim that it causes cancer at low levels. The summer tradition started back in the 1980s with the belief that minuscule amounts of pollutants, what fretful people call “air toxics,” cause cancer. How could anything “toxic” not be cancerous?
Scientists now envision that more common air pollutants — nitrogen dioxide, carbon monoxide and fine particles — cause cancer based on studies that are mostly wild searches of correlations. Today, they speculate that various outdoor air pollutants are linked to the following cancers or cancerrelated deaths: bladder, bowel, brain, breast, cervical, colon, kidney, leukemia, liver, lung, nose, prostrate, rectal, skin, stomach, throat and non-hodgkin lymphoma. Policy-makers at Health Canada and Alberta Health Services have unfortunately embraced some of their speculations.
Despite the speculation, it is extremely unlikely that air pollution causes all sorts of cancers given the importance of established causal risk factors for this disease. A causal risk factor is something sufficient to produce a condition, in this case cancer. Multiple risk factors are what really contribute to cancer. Interaction of these factors is complex.
The air pollution-cancer link can be disproved by looking closer at these risk factors. Here it is necessary to sort through evidence that relies on epidemiology and toxicology, which are not exact sciences. It is not that hard if we focus on scientific methods, evidence and uncertainties. Just ignore scientists’ and policy-makers’ interpretation of the evidence, which is more often based on art than science.
Proximal risk factors are those close to a condition. Evidence on proximal risk factors for cancer is well established with uncertainties well understood. They are age, genetic background, biologic (ethnicity, blood type), lifestyle (unhealthy diet, physical inactivity, to- bacco use, alcohol misuse), environment (sunlight, certain infectious agents), certain medications, and certain workplace exposures (although most are likely not important today).
Distal risk factors have distant causal influences. Evidence from studies of distal risk factors faces challenges in meeting criteria for being causal factors for cancer. Why is this? For many factors there is uncertainty about how they actually cause cancer. Many studies use weak or biased scientific methods or the populations studied only have subtle or poorly understood measures of exposure. Finally, their evidence is generally weak. As most of these studies are exploratory, their evidence is only suggestive and hence the need for speculation. Examples of distal risk factors are everything from coffee, tea and wine to cosmetics, cellphones plus other flimsy suspects including outdoor air pollution.
How flimsy is outdoor air pollution as a cause of cancer? Looking closer at aging, adults are internally exposed to about 500 grams of oxygen daily from breathing air. Oxygen is both a friend and foe. It is essential for our survival. However, intracellular oxygen abets many pathological processes through oxidative stress, which induces cell DNA damage. Subsequent DNA replication errors are key to cancer. For all outdoor air pollutants combined that are speculated to cause cancer, their total from breathing air is trivial — less than 0.00001 grams for every gram of oxygen taken up over an adult’s lifetime. Such a small internal exposure compared to oxygen is meaningless. Also consider that the other proximal risk factors are well known for producing oxidative stress.
Outdoor air pollution is a fake risk factor for cancer. Speculation about air pollution causing cancer should come with a warning: Taking advice from scientists and policy-makers about air pollution can be harmful to your health. They believe breathing outdoor air causes cancer. Please use common sense (ignore them), or breathe air responsibly.