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The Fly in the Ointment
70 Fascinating Commentaries on the Science of Everyday Life
By Joe Schwarcz
ECW PRESSCopyright © 2004 ECW Press
All rights reserved.
Lies, Damned Lies, and Statistics
I overheard an interesting conversation between two young women as I was waiting in line to ride The Comet, the granddaddy of roller coasters at Great Escape Fun Park in upper New York State. One was preparing to study in Australia and was describing her travel plans. Her friend thought Australia would be exciting but added that she would never go herself because flying was too dangerous. The prospective traveler responded that she wasn't concerned about a plane crash but was worried about the risk of developing deep vein thrombosis—a potentially fatal blood clot—during the trip. As soon as I heard this comment I knew that she must have watched the same TV talk show I had the day before.
While the specifics of this conversation might have been unique, the gist of it was not. Details aside, the young women were involved in risk evaluation, something we all do on a regular basis. Just think about how often we ask ourselves whether or not we should be worried about mercury in tuna, radiation from cell phones and microwave ovens, aspartame in diet drinks, and the reported link between estrogen supplements and an increased risk of cancer. Life often comes down to analyzing risks. But most people do not realize how difficult it is to perform this analysis in a meaningful way.
Let's start with something easy, like the risks of air travel. Flying is actually remarkably safe. Since the advent of commercial air transport around 1914, some 15,000 people have perished in airplane crashes. In North America alone more than three times that many people die in automobile accidents every year! You are far more likely to arrive at your destination if you fly than if you drive. Unfortunately, traffic on the highways increased significantly after September 11, 2001, resulting in many deaths that would not have occurred if people had flown. So why are people so scared of flying? Because they don't think statistically—they think emotionally. People have the perception that their destiny is in their own hands if they are driving a car; they feel that they have relinquished this control when flying on an airplane. Also, there is a greater likelihood of surviving a car crash than a plane crash, which is another factor that weighs on people's minds. But the statistics show that over a lifetime, you are 100 times more likely to die in a car accident than in a plane crash. Basically, you are more likely to be struck by lightning or win the lottery than die in an air disaster. Flying to Australia is safer than driving to the Great Escape from New York City, which is what the two young women had done.
Now, about the deep vein thrombosis. The TV show I had seen focused on what has come to be called "economy-class syndrome" and began by recounting the tragic case of a healthy British woman in her late twenties who collapsed at Heathrow Airport in London after a long flight from Australia. She died within hours from a blood clot in her lung that had originally formed in her leg while she sat in a cramped position for an extended period. The show also included interviews with physicians in Hawaii who described similar incidents. There is no question that such deep vein thrombosis can occur, but the number of people who develop this condition is very small when compared with the number of passengers that fly. Indeed, a study reported in the New England Journal of Medicine found no association between air travel and deep vein thrombosis. On long flights, passengers should certainly be encouraged to move around, particularly if they are seniors, have a history of heart disease, are pregnant, or are taking estrogen supplements. The risk of deep vein thrombosis on a flight is a minute statistical blip, but if you take three victims and put them on a talk show together, viewers will think that the air travel industry is in midst of an epidemic.
If you want something to worry about on a flight, worry about the air quality. Although the recycled cabin air is filtered and mixed with fresh air, during flight many passengers complain of nausea and flu-like symptoms, which are consistent with a reduced oxygen supply. Of even greater concern is the spread of infectious organisms within the confines of an airplane cabin. In one documented case, passengers were kept on board while a plane underwent a minor repair. One of the passengers had a case of influenza A, which spread to three-quarters of the other travelers.
The way data are communicated can also affect people's perception of risk and the decisions they make. Take, for example, the recent study that showed a 30 percent increase in risk of breast cancer among women taking estrogen supplements. Sounds terribly frightening! But consider that within a ten-year period, about 3 to 4 percent of menopausal women will be struck by breast cancer. A 30 percent increase in this risk means that that number rises to 5 percent; suddenly the 30 percent increase in risk doesn't seem quite as impactive. Putting it another way, a postmenopausal woman who takes estrogen reduces her chance of remaining cancer-free from about 96 percent to 95 percent.
Consider also this example: A pharmaceutical company's ad, aimed at physicians, touted a drug's ability to reduce the risk of heart attack in a group of patients by 25 percent. In the wake of the ad, the number of prescriptions being written shot up. In reality, however, the drug study showed that 0.4 percent of patients had fatal heart attacks when not taking the drug as opposed to 0.3 percent of those who did take it. When statisticians examined the data they concluded that if more than seventy patients were treated with the drug for five years, the treatment would help just one of them avoid a heart attack. With results stated in this way, the drug sounds far less appealing.
And how about this: Benzopyrene in charcoal-broiled foods is a known carcinogen, because in large doses it will cause cancer in rodents. Based on the animal model, scientists estimate that eating 100 charcoal-broiled steaks will increase the risk of cancer by 1 in 1 million, a number that has arbitrarily been selected as a "red flag." What does this statistic really mean? Since the risk of cancer over a lifetime is about 1 in 3, eating these steaks will raise that risk by 0.0003 percent. It makes more sense to limit consumption of steaks because of their fat content, not because "they cause cancer."
It would have been fun to engage the two young women at the Great Escape in a discussion of risks. I could have told them about the sixty-four-year-old man who developed "shaken baby syndrome" after riding a roller coaster. Given that the two were smoking like steam engines, I also would have liked to mention that every 1.4 cigarettes increased their risk of cancer by 1 in 1 million. But with all that secondhand smoke around, I didn't want to venture any closer. Risk analysis is a risky business.
Farmed, Wild, or Canned?
It's a pretty common scenario these days: Scientists publish a paper about the presence of a synthetic pollutant in a consumer product and warn people about excessive exposure because the substance is known to cause cancer or reproductive problems when fed to rodents in high doses. Said study about yet another cancer-causing substance in the environment makes front-page news. Spokespeople for the industry in question complain bitterly that the risk has been exaggerated while environmentalist groups hail the study as a breakthrough. Scientists with impeccable credentials wade into the debate on both sides, sometimes accusing each other of having vested interests. Different government regulatory agencies can't agree on what recommendations to make. The public is thoroughly confused, and my office gets lots of e-mails and phone calls.
A prime example of this scenario is a scare triggered by a paper published in the prestigious journal Science in 2004. In it, researchers reported that farmed salmon are significantly more contaminated with organochlorine compounds such as PCBs, dioxins, toxaphene, and dieldrin than their wild counterparts. PCBs were once commonly used as insulating fluids in electrical equipment, dioxins are by-products of some industrial processes, and toxaphene and dieldrin are insecticides. These chemicals are particularly persistent in the environment and are fat-soluble. As a result, they accumulate in the fatty tissues of farmed fish, which are fed fish meal and oil made from smaller fish that also contain deposits of these chemicals. Similarly, when we eat contaminated fish, the organochlorides can build up in our fatty tissues. Everyone agrees that this buildup is not a good thing. Why? Because there is evidence that these compounds are capable of producing some pretty nasty health effects.
Let's use PCBs as an example and examine the related cancer risk. There is no question that PCBs can cause the disease in animals, with the liver being the main organ affected. The human picture is less clear. Epidemiological studies have shown that workers with extensive exposure to PCBs in an industrial setting suffer a slightly elevated risk of cancer. Some investigators have also found a significant association between PCB concentrations in fatty tissue and non-Hodgkin's lymphoma. A couple of incidents in Japan and Taiwan, in which people ingested rice oil accidentally contaminated with a high dose of PCBs, are also suggestive of an increased risk of liver cancer. Labeling PCBs as probable human carcinogens therefore seems justified. As we have seen, PCBs are present in fish, particularly in the farmed variety. But that does not mean that eating fish raises the risk of cancer. Our food supply contains numerous carcinogens, both natural and synthetic. Hydrazines in mushrooms, heterocyclic aromatic amines in cooked meat, aflatoxins in molds, and acrylamide in baked goods are all carcinogenic. But our diet also contains anticarcinogens in the form of various vitamins and polyphenols. When we eat we consume hundreds of different chemicals, and the result of their interplay in our body is virtually impossible to predict. That's why the appropriate question to ask is not whether organochlorine contaminants in fish can cause cancer, but whether a diet high in fish can do so. I am unaware of any study that shows a link between increased fish consumption and cancer. On the other hand, numerous studies point to just the opposite conclusion!
Swedish researchers have clearly shown that eating fatty fish, salmon in particular, can reduce the risk of prostate cancer by one-third. Italian and Spanish scientists have investigated the relationship between frequency of fish consumption and cancer and found that for those who consumed fish regularly, there was a consistent pattern of protection against the risk of digestive tract cancers, particularly of the colon, one of the leading causes of cancer mortality in developed countries. At the Aichi Cancer Centre Hospital in Japan, scientists looked at the diets of more than 4,000 healthy people and another 1,000 with lung cancer. Both men and women who ate large amounts of fresh fish were significantly less likely to develop lung cancer. This finding may explain why the Japanese, who smoke more than Westerners, have a lower rate of lung cancer. An extensive survey conducted over ten years, involving more than 60,000 people of Chinese descent in Singapore, found that women who eat at least 40 grams of fish per day reduced their risk of breast cancer by 25 percent. There is sound theoretical justification for these observations. Prostaglandins are a class of chemicals in the body that produce a variety of hormone-like effects, some of which are linked to carcinogenesis. These chemicals are derived from arachidonic acid, which in turn is formed from linoleic acid, a common omega-6 fat in the diet. Fish oils inhibit the cyclooxygenase-2 enzyme that converts arachidonic acid to the problematic prostaglandin E2. Essentially, reducing fish intake is likely to result in more—not less—cancer, irrespective of the contaminants fish may contain.
While the prospect of cancer instantly strikes fear into the heart, the fact is that strokes and heart disease kill more people. And there is overwhelming evidence that links fish consumption with protection from strokes and heart attacks. But why stop with cancer, strokes, and heart disease? Recent evidence indicates that fish consumption offers protection from diabetes and maybe even from Alzheimer's disease. In all cases, the beneficial chemicals are believed to be the omega-3 fats, of which salmon may be the richest source. In fact, farmed salmon, on average, contain somewhat more omega-3s than their wild counterparts. Furthermore, salmon—farmed or wild—is less likely to be contaminated with mercury than other commonly eaten fish. So what you want to ask yourself is whether you should put more emphasis on the theoretical risks associated with organochlorides in fish or the proven benefits of fish consumption.
Although the answer to this question should be obvious, the salmon study in Science is still an important one. It will undoubtedly encourage fish producers to take steps to reduce the organochloride residues in their product, something that is technically feasible. The use of feed made from canola and soy oil genetically modified to contain more omega-3 fats is an interesting possibility. Incidentally, canned salmon almost always comes from wild Alaskan salmon, which are minimally contaminated with organochlorides. Most fish oil supplements, of which the usual recommended dose is 2 to 4 grams per day, are also free of these compounds.
While the authors of the Science paper deserve credit for compiling valuable data on salmon contamination, I believe their argument—which is that more than one meal of farmed salmon a month may hike the risk of cancer—is totally unjustified. Indeed, I think it can be effectively argued that any such cutback in salmon consumption is seriously detrimental to health. Since wild salmon is quite expensive, the warning about farmed salmon could have the effect of significantly reducing salmon consumption in the population, thereby increasing the risk of illness. I would agree, though, that it is a good idea for pregnant women to stick to wild salmon, just to be ultrasafe.
As far as I'm concerned, based on the studies I examined while looking into this issue, I would happily keep eating a couple of servings of fish per week—farmed or wild. Alas, I'm allergic to fish, so I will have to stick to my flaxseed, which is a far poorer source of omega-3 fats than salmon. I'm also told that it doesn't taste nearly as good.
DDT: A Double-Edged Sword
I remember being quite taken when, as a student, I read Rachel Carson's 1962 epic Silent Spring. As a biologist, Carson made a compelling case against the synthetic pesticides that had been introduced in the post–World War II era. She maintained that they were responsible for fish kills, pollution of the soil, and reproductive problems in birds. DDT in particular caused thinning of egg shells and led to fewer hatchings. Ospreys, peregrine falcons, and eagles were disappearing, Carson said, and robins were being killed in misguided attempts to eradicate Dutch elm disease by spraying trees with DDT. That's why there would eventually be no birds to sing: there would be a "silent spring."
I was impressed by Carson's book. I thought it was an excellent example of how we cannot always predict the consequences of a chemical intervention and how the introduction of a substance into the environment, although seemingly for all the right reasons, can backfire. Carson made an impassioned plea against putting blind faith in technology, particularly when it came to pesticides such as DDT.
This notorious compound was first synthesized in 1874 by Othmar Zeidler, who combined chloral (which later became known as a "Mickey Finn" after the Chicago bartender who supposedly used it to put his rowdy patrons to sleep), chlorobenzene, and concentrated sulfuric acid to make it. Zeidler was simply interested in making novel compounds for his Ph.D. thesis and never studied DDT further. But in 1939, Paul Muller, working for the JR Geigy Company in Switzerland, did. He was interested in moth repellants and had come across a compound called "diphenyltrichloroethane," which was somewhat effective. Muller then did a literature search and came upon DDT, a closely related substance. He synthesized it according to Zeidler's recipe and discovered that it was remarkably toxic to insects. And much to his satisfaction, it seemed not to have any effect on domestic animals or humans. Swiss farmers were thankful. Just a year after Muller's discovery, DDT was used to wipe out the Colorado potato beetle, which had threatened the country's potato crop.
By 1945 DDT was being used worldwide on numerous crops. But concerns arose with two discoveries: the chemical's application caused it to disperse into the air and spread far and wide, and it was showing signs of toxicity in frogs and fish. By the 1950s it was apparent that DDT was building up in the fatty tissues of animals and humans. Eventually, the US Environmental Protection Agency (EPA) stepped in and banned the substance. Rachel Carson had played her role, the environmental movement had begun, and a major problem had been eliminated.
Excerpted from The Fly in the Ointment by Joe Schwarcz. Copyright © 2004 ECW Press. Excerpted by permission of ECW PRESS.
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