Monday, January 15, 2007

Ask A Scientist: Is Microwaving Plastic Wrap Bad? (2)

Part 1: Plastic Wrap and Dioxins

Part 2: Phthalates

[With correction below.]

2. Phthalates: In short, you may get small doses from microwaved plastics; but you’ll probably die of something else, eventually.

Phthalates are known as ‘endocrine disruptors’, which means they can screw up your hormone system. Since hormones control everything from reproduction to disease response (inflammation and immune function), this can be very bad. Phthalates share a common structure (left), where ‘R’ is any chemical group, but usually a carbon chain or a benzene ring in this case. They are widely used as plasticizers, which in general impart some desired characteristic to the plastic by modifying the physical properties of the polymer base: flexibility, strength, color, etc. Many modern plastics have plasticizers in them; they have helped lift us out of the Bakelite era.

Phthalates in general bear a passing resemblance to estrogen (left: estradiol). DEHA (below left) is the phthalate most commonly worried about in plastic wrap. There is a whole group of these chemicals, with annoying names; mainly, the ones beginning with D have two ‘R’ groups, and the ones beginning with ‘M’ are usually metabolites of the D’s, in medical studies.

First the bad stuff: if you give a rat about 2.2 g/kg/day (1/500th of its body weight! For an average human, about five ounces a DAY) of butyl benzyl phthalate, you get serious bad effects on the male reproductive system. Lower doses, 20 or 200 mg/kg/day, produce some mild anemia and small changes in blood cell distribution.

A study by Swan et al. started much of the (scientific) fuss. The ‘ano-genital distance’, or AGD, which is what it sounds like, happens to differ inherently for male and female rats; same with humans, apparently. Swan and colleagues measured the AGD and a few other parameters (height, weight) and got urine samples out of 85/134 mothers. Then they did a bunch of fancy statistics.

Their conclusions are that 1) AGD is a measure of 'feminisation'; this is supported only by animal studies, and by AGD being different in male/ female humans and 2) phthalates are terribly, awfully dangerous because who knows what else could be happening; this is based on the assumption that AGD is an indicator of, well, anything.

There are many criticisms of this paper; I, for one, find it deeply unconvincing.

If you’re interested, look at Figure 1.They want to assert that there’s a large difference between the red dots and the blue dots. But they only have about ten kids each in the high/ low categories, so the data overlap a lot, plus the fit of AGI vs. age was messy to start with (R2=0.61, or 61% of the data can be explained by the fit). All their significant differences are between fit coefficients, not groups- that is, the data themselves when grouped don't have any real differences, but if you draw a 61%-accurate curve through the groups and compare the curves, you see a 'significant' difference.

Other people have criticized the phthalate testing in this study for not being normalized to sample volume (did the mothers drink a lot of water that morning?), for failing to take into account other factors that affect development, and for sampling wantonly across a broad age range. With such a small number, it is almost impossible to come up with true correlations in such complex processes. In other words: It’s like looking at a classroom and trying to figure out if the kids who are wearing Velcro sneakers are more likely to eat celery. You have no idea what else influences them, and you’re not entirely sure the two things are related, and you don’t have a big enough sample to tell anyways.

The authors claim that perhaps humans are very, very sensitive to phthalates: much more than rodents- and this accounts for lowering! sperm! counts! in men! worldwide crisis! Aaaaah! But the levels found in these human mothers are about 1 ug/kg/day ingested; that is, two hundred thousand (200,000) times less than the rats who had problems.

There is an excellent review on the possible effects of phthalates, and a summary of animal studies.

Overall, it seems like the levels of possible exposure from plastics are very low. In addition, Saran Wrap has no phthalates as plasticizers. It is likely that high exposures can cause fertility problems, and there are studies indicating that high levels can inhibit tamoxifen (i.e. screw up your breast cancer treatment, maybe). There is not good evidence that low levels will do any harm, and even if they do, your water is coming in PVC pipes; why are you worrying about the microwave? If it is a cancer risk, it is a low one; I imagine on par with eating well-grilled hamburger. There is no way to eliminate all risk.

So in brief:
  • There may be some phthalate exposure from microwaving plastic.
  • Everything else plastic in the world has phthalates in it. All your food has probably touched phthalates.
  • Phthalates may or may not be harmful in food-plastic-related doses.
  • Overall lifetime risk of cancer is high from all causes, because cancer is a disease of age. You have to die of something eventually. Have another hamburger.
  • If you are really concerned about plasticizers leaking into your food: put new plastic containers in hot water with soap and soak overnight. This will probably remove whatever is water-soluble. Repeat with oil if you're feeling fanatical.
  • If you’re really really worried about it, use glass, ceramic, and metal to replace all plastic containers in your life. But this will probably reduce your total exposure minimally.
Correction: Astute reader Dan L. kindly writes to tell me that PVC often has plasticizers; other plastics like HDPE, polypropylene, and polystyrene are generally made without plasticizers. To quote the ever-thrilling journal "Progress in Polymer Science" (doi:10.1016/j.progpolymsci.2004.10.001):
The most frequently plasticized polymers include PVC, poly(vinyl butyral) or PVB, poly(vinyl acetate) or PVA, acrylics, cellulose molding compounds, nylon, polyamides and certain copolyamides. On average, PVC accounts for about 80% of all plasticizers consumed [11] (88% in Europe and 85% in North America [2]).
Polystyrenes are sometimes plasticized for uses as yet unknown to me; also in fire-fighting gear.
The Canadian Conservation Institute cites several examples of commonly used polypropylene and styrene tubing that include plasticizers, as well.

[Note: I am not a medical professional; this should not substitute for medical advice or treatment. This is intended as an academic summary of the evidence available. For medical needs, please consult your physician.]