9/21/04

Pollution and cancer in China

The recent article in the Times is harrowing, starting from its title. But the scientific questions it raises are just as important as the sympathy and disquiet it evokes. There are two issues here. First, what is the relationship between the recent increased pollution in rural China and cancer rates (or death rates due to cancer) in those regions? Secondly, apart from the epidemiologic facts, why are the people interviewed in the article so sure that they have cancer?

A brief, incomplete survey of the epidemiologic literature (oh, let me come clean here: I just went to Medline, typed in "China+epidemiology+pollution," and read a bunch of abstracts. It's Google for epidemiologists!) indicates both rather more and less than is pointed to in the Times. The rural residents complain of gastric and other GI cancers, and the available literature (much of which is, not surprisingly, available only in translated abstracts of Chinese-language articles) points to increased rates of esophageal cancer after exposure to polluted water. (I should note that the tricky part of epidemiology is figuring out the contribution of many different factors. "Correlation does not imply causation" might be tattooed on the body of every epidemiologist.)But there are also a number of articles which point to a broader array of deleterious effects, from nasopharyngeal cancer to hepatitis B infection (which can lead to primary liver cancer). A few studies, as well as the Times article, mention what would seem to be a more acute endpoint: frank toxicity due to astronomical levels of various toxins, effluents, and general nasty chemicals in well- and riverwater.

The Times article implies that the rise in cancer rates due to pollution is a recent phenomenon, of the past couple of decades. I think this is true in a sense, but the sense was corrupted due to necessary abridgment for journalistic purposes. The course of events probably goes something like this: over the past few decades, maybe since the economic liberalization that was allowed during the 70s and 80s (my Chinese history is weak, but I think this is right), industrial activity has produced and allowed widespread pollution. This pollution has indeed led to increased cancer among rural Chinese -- but only because rural Chinese are living longer (due to improved nutrition and hygiene) and are thus able to develop cancer during their additional lifespan. In these polluted rural areas, it's only over the past decade or so that these cancer cases have reached a critical mass that can't be ignored.

Another possibility is that these diseases among rural Chinese, though probably to be laid at the feet of polluters (and the Chinese public-health establishment, necessarily weakened by totalitarianism -- cf. the recent SARS mess), are also caused by simple toxicity due to the ingestion of a high concentration of pollutants.

This is just rank speculation, of course, but it's speculation that I would bet Chinese public-health workers and epidemiologists have to engage in, to decide whether cancer is, in fact, pandemic in rural China and what to do about it (besides, obviously, cracking down on pollution).

* * *

It's not clear from the article in the Times that the people interviewed do in fact have cancer. (They are horribly ill, but it's not apparent from what.) I think that in some cases they might be extrapolating from changing causes of death among people they know. As the Chinese get richer, their causes of death may shift from infectious diseases (with which many rural Chinese may already be acquainted) to the so-called "diseases of affluence" -- heart disease, stroke, and cancer. As it turns out, the Chinese Red Cross recently released a study of widespread ill-health among urban Chinese. [Strangely enough, the study isn't mentioned on the CRC's Web site. Maybe it's on the Chinese site -- can anyone tell me?]

* * *

China's the most populous country in the world, with huge populations that are urban, rural, urbanizing, or migrating from city to country. I hope China deals with its public health problems in a way that will serve as a model for other systems.

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