I drink water from my plastic cup regularly and a friend commented that it could cause cancer.
He also added that I should not heat food in plastic containers or wrappings in the microwave as that could also cause cancer in the long run. He claimed that this is due to the plastic slowly entering what you are consuming when heated or by simple erosion.
How much of this is true and supported by scientific evidence?
It's going to depend upon the type of plastic you're using, as they're composed of different chemical compounds. If you look at the bottom of your plastic cup (or dish, etc), there's normally a recycling symbol and a number. The number classifies the type of plastic it is.
You can find the plastic classification numbers here.
My opinion as an environmental chemist: if your drink cup is #2, #4 or #5, then your risk of exposure is low enough to not be an issue, as these classes of plastics do not normally contain bisphenol A (BPA) or phthalates.
You should not be using #3 (polyvinyl chloride aka PVC) for anything food related; they contain bisphenol A and phthalates; both of which are endocrine disruptors.
No. 6 is fine for storing foods, but you should not reheat foods in polystyrene. No. 7 is a 'catch all' for plastics - some of these can contain BPA.
It is nearly impossible to be completely BPA/Phthalate free; they're nearly ubiquitous in our lives: plastic wrap, the coating inside your canned beans, etc.
There are reports which suggests that BPA causes cancer, and there are many published studies regarding phthalates impacting sperm count and quality. BPA has been tentatively linked to premature on set of puberty.
Before you run off and throw out everything plastic in your house, you need to understand that you're exposed to BPA from more than just leaching from your drinking bottles & cups. BPA will degrade rapidly in air but it persists in water - this means that you're potentially exposed to BPA via your drinking water (if it's a public water supply) or through what you eat (fish) since BPA is both bioaccumulated and biomagnified.
So yes, there is a risk of BPA/phthalate leaching from your cup (or container) to your water (and thus into you), however the amount leached is most likely very small. In most of the studies where laboratory rats developed cancer, they were receiving large (1000-10000 ppm) doses of BPA through their food, or via subcutaneous injections.
It's why they pushed to have BPA and phthalates removed from baby bottles (which are still normally marked with a 7 code!) - there was a higher risk of BPA/Phthalate exposure to infants, who are normally given their milk warm. Higher dose given + consistent exposure = increased risk.
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).
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 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" 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, but many regulatory bodies like the EFSA and FDA consider the current allowed levels safe. Although recently the EU banned BPA from baby bottles.
No, there is no credible risk of cancer or other adverse effects from oral consumption of BPA.
The studies that claim BPA might pose a danger have methodological issues. STATS.org of George Mason University notes:
The European Food Safety Authority (ERAS) - said that the amount considered safe to ingest on a daily basis for life should be raised by a factor of five.
Many of the studies that show adverse effects in rodents given small doses of bisphenol A subcutaneous injections. Most of the studies in rodents that did not show adverse effects even in high doses used the oral route.
there is "a 500,000 fold difference between the lowest oral exposures in animals associated with any adverse effects and the oral human exposure," [...] By any measure, this does not constitute a health risk. We get vastly more estrogenic chemicals from eating nuts, cereals and bread.
