This claim is most likely related to the presence of certain controversial chemicals present in some kinds of plastic. The most discussed is certainly [Bisphenol A (BPA)][1].

![enter image description here][2]

The claim about microvaving in plastic bottles is probably related to the fact that BPA leeches faster out of plastic with hot water than with cold water. [One study][3] found that

> Exposure to boiling water (100 °C)
> increased the rate of BPA migration by
> up to 55-fold

Bisphenol A can mimic estrogen and thereby cause a variety of health effects.

In the review ["Environmental causes of cancer: endocrine disruptors as carcinogens"][4] the authors state the following about the occurence of Bisphenol A:

> The xenoestrogen BPA is one of the
> EDCs that has been most thoroughly
> studied. BPA is found in various
> consumer products including baby
> bottles, reusable water bottles and
> reusable food containers, polyvinyl
> chloride stretch films, papers,
> cardboards and in the epoxy resins
> lining the insides of food cans.
 
On the carcinogenic properties of BPA they state:

> Rats exposed prenatally to
> environmentally relevant doses of BPA
> show an increased number of
> intraductal hyperplasias (precancerous
> lesions) that appear during
> adulthood, while high doses
> induce the development of carcinomas
> in situ.

There is still some conflict about the interpretation of the studies. [The Endocrine Society is concerned][5], but many regulatory bodies like the EFSA and FDA consider the current allowed levels safe. Although recently [the EU banned BPA from baby bottles][6].


  [1]: http://en.wikipedia.org/wiki/Bisphenol_A
  [2]: https://i.sstatic.net/NJzxa.png
  [3]: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCR-4R5F02G-1&_user=10&_coverDate=01/30/2008&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1719805448&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8f03955e703b623c6142c69b3f3de647&searchtype=a
  [4]: http://www.nature.com/nrendo/journal/v6/n7/full/nrendo.2010.87.html
  [5]: http://www.endocrinetoday.com/view.aspx?rid=40865
  [6]: http://www.physorg.com/news/2010-11-europe-baby-bottles-bisphenol-a.html