PVC is stable enough in heat to be used for pipes for hot water (even in health-conscious Europe). Bong water and water-filtered smoke are not nearly as hot as this. The organic plasticizers not tightly bound to the PVC are lipophilic, and would not tend to dissolve in bong water or smoke.
Perhaps this rumor got started because of the fact that when PVC itself is combusted, toxic carcinogen- and irritant-containing smoke is produced. But as long as the bowl and stem are not made of PVC, none of the PVC will be heated significantly, much less combusted.
I stick with my original contention that plastic pipes are not clean, health wise. The plasticizers mimic the action of sex hormones at very small concentrations, rather than high doses. This does cause deformations and cancers to human organs and cells.
In a recent report entitled “Sick of Dust, Chemicals in Common Products” by the Safer Products Project (www.saferproducts. org, Tel. 718-805-1056, POB 153, Spring Brook, NY 14140) results of tests on household dust showed that most households were contaminated with six or seven powerful chemicals. One group, the “organotins” are used as heat and light stabilizers or “plasticizers” in PVC products such as tubes and pipes. They are released from the plastic upon heating. Here is what the research organization said of this class of chemicals: “Very poisonous even in small amounts, disrupts the hormone and reproductive system; toxic to the immune system.”
You mentioned that the plasticizers used in PVC are lipophilic. That means they dissolve in fat and can build up in body tissue.
Plasticizer release from burning and heated plastic is the main problem. The point where the metal bowl meets the plastic pipe is a possible heat site. There are also accidents when using all tools. We have all seen partially melted pipes. These definitely suffer from plasticizer release.
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