Sunday, April 17, 2011

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A Shuletvitzian nightmare

"Intuitive Toxicology" and Panicking Over Plastics at The New Republic

Over at The New Republic journalist Judith Shulevitz is wringing her hands over "The Toxicity Panic." Actually, she seems to be doing her best to fuel the panic. The chief agents of terror are everyday plastics and other products, especially bisphenol-A and phthalates. These synthetic chemicals mimic or interfere with hormones like estrogen and testosterone and are known as endocrine disruptors. A worried Shulevitz suggests that untested modern chemicals are responsible for all kinds of diseases:

By now, you may be asking, if our health is so sensitive and if we live in a total plastic environment, why aren’t we sicker than we are? And sicker than we used to be? The answer is, we’re healthier in some ways and sicker in others. Medical advances mean we’re likelier than ever to survive our illnesses, but all kinds of diseases are on the rise. Childhood cancers are up 20 percent since 1975. Rates of kidney, thyroid, liver, and testicular cancers in adults have been steadily increasing. A woman’s risk of getting breast cancer has gone from one in ten in 1973 to one in eight today. Asthma rates doubled between 1980 and 1995, and have stayed level since. Autism-spectrum disorders have arguably increased tenfold over the past 15 years. According to one large study of men in Boston, testosterone levels are down to a degree that can’t be accounted for by factors such as age, smoking, and obesity. Obesity, of course, has been elevated to the status of an epidemic.
That does sound terrible. But in the next paragraph Shulevitz admits that there could be other factors accounting for the rise in the diseases she mentions:
There are many ways to explain upticks in rates of any particular ailment; for starters, a better-informed populace and better tools for detecting disease mean more diagnoses. Other environmental stressors include Americans’ weirdly terrible eating habits, our sedentary lifestyle, and stress itself.
Well, yes. But let's take a closer look at the specific diseases she mentions. What about childhood cancer rates? Childhood cancer incidence has indeed increased. In 1975, the rate was 11.5 cases per 100,000 children, ages 0-14. The most recent figure (2007) is 15.5 cases per 100,000. There was a big jump in the rate of childhood cancer between 1975 and 1985, when it was 14.5 cases per 100,000. The National Cancer Institute reports:
The causes of childhood cancers are largely unknown....Environmental causes of childhood cancer have long been suspected by many scientists but have been difficult to pin down, partly because cancer in children is rare and because it is difficult to identify past exposure levels in children, particularly during potentially important periods such as pregnancy or even prior to conception.
What about kidney, thyroid, liver, and testicular cancer rates? Yes, they have been going up too. But Shulevitz is engaging in a bit of cherry picking when it comes to cancer trend data. The latest report from the National Cancer Institute finds that the overall cancer incidence rate in the United States has been declining by about one percent per year for more than a decade. For men, the data for the big three killers, prostate, lung, and colon cancers, show annual declines respectively of -0.3 percent, -1.9 percent, and -3 percent. In fact out of the 17 most common cancers, incidence rates for men declined or remained flat for 11 them. For the record, the incidence rank of kidney, thyroid, liver, and testicular cancer for men is respectively 7, 12, 18, and not even listed.
For women, the overall annual incidence rate of various cancers has also been declining. The breast cancer rate has been going down at -1.2 percent per year since 1998. Colon cancer down by -2.3 percent, and ovarian cancer by -1.8 percent. Out the top 18 cancers that afflict women incidence rates have declined or remained flat for 12 of them. It's worth noting that decline in breast cancer may well be related to the recent rapid cessation of hormone replacement therapy. Thyroid, kidney, and liver cancers rank 6, 9, and 18 for women.
And what about asthma rates? Asthma rates did about double between 1980 and 1995 and have since leveled off. But roots of the increase are not clear. Many researchers believe that the hygiene hypothesis can explain higher levels of asthma in modern societies. Some research has shown that growing up in relatively sanitized environments prevents a child's immune system from developing properly which leads to the increased likelihood allergies and asthma. Early exposure to microorganisms may protect against asthma. In fact, asthma rates do go up as countries become richer. Of course, while the environments of people in developing countries are becoming cleaner they are also being exposed to more plastics. In any case, other research suggests that the hygiene hypothesis may not be all there is to the asthma story.
Autism-spectrum disorders certainly do appear to be on the increase. Today about 1 child in 110 is diagnosed with some form of autism. Why the increase? The vaccine/autism hypothesis has been soundly rejected. A 2009 review of autism rate statistics by Eric Fombonne at McGill University notes:
There is evidence that the broadening of the concept, the expansion of diagnostic criteria, the development of services, and improved awareness of the condition have played a major role in explaining this increase, although it cannot be ruled out that other factors might have also contributed to that trend.
Earlier research found that as public schools began to offer more services to children diagnosed as autistic, the diagnosis increased substantially.
And what about that Boston study that found men's testosterone levels were falling? It is true that study tried to rule out confounders like smoking and obesity, but later research suggests that obesity may well play a role:
Current evidence suggests that body composition changes as expressed by BMI can in part account for the trend in testosterone. More speculative recent findings suggest a potential contributory role for environmental endocrine disruptors, but to date no longitudinal studies have examined this question.
Note that the researchers speculate that endocrine disruptors may be a factor, but have no evidence for it. A recent review of clinical studies finds no deleterious trends in the prevalence of congenital abnormalities in male sexual organs and sperm counts.
Which brings us to obesity. Why are Americans becoming fatter? Could plastics be at fault? Maybe, but here's a simpler hypothesis: We're eating too much and exercising too little. The U.S. Department of Agriculture says that
... dietary intake of calories in 2000 [is] at just under 2,700 calories per person per day. [Economic Research Service] ERS data suggest that average daily calorie intake increased by 24.5 percent, or about 530 calories, between 1970 and 2000.
Since 2000 our calorie intake has been stable at this relatively high level.
Interestingly, obesity correlates with a lot of the diseases that Shulevitz is worried about: Kidney cancer, thyroid cancer, liver cancer, breast cancer, and asthma. Obesity apparently protects against testicular cancer (although being tall is a risk factor), but it is negatively correlated with testosterone levels. While there's no evidence that obesity causes cancer in children, there's lots of research that suggests that being a fat child increases one's risk later in life.
As Shulevitz points out tests for the presence of substances in the body have gotten vastly more sensitive so researchers are finding more stuff all the time. A 2009 review of major biomonitoring studies found that average exposures to bisphenol A is about one-thousand times less than the levels determined by the EPA to be safe for daily exposures. Similarly, the study found the average exposures for the general population to phthalates are below levels determined by the EPA to be safe for daily exposures.
In addition, researchers can now see how various compounds affect the operations of thousands of genes at a time, although knowing what health effects they may be causing is still quite murky. In the future toxicogenomics may become a very useful tool for screening compounds for health risks, but the field still has some way to go before being added to the armamentarium of regulatory science. In the meantime, Shulevitz apparently thinks we should rely on our intuitions, which is all very well, but arguably should not be the basis for regulatory decisions.
A couple of final correlations: As our use of plastics has increased so too have our life expectancies, rising from 68 years in 1950 to 78 years now. And we're not only living longer, we're living longer with fewer disabilities. And while we're relying on our intuitions, my intuition is that the hypothesis that trace exposures to endocrine disruptors are responsible for a lot illnesses will prove to be as overblown as the hypothesis that exposures to trace amounts of synthetic chemicals is a major cause of cancer. Panic over plastics? Not me.
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