Last week, hard plastic baby and water bottles were not considered harmful.

Now, in the eyes of many users, they are toxic. Yesterday, CVS said it will join Wal-Mart , bottle-maker Nalgene, and other companies in pulling tens of thousands of the shatter-proof, transparent products off store shelves. Some parents are tossing hiking bottles into the trash, feeding their babies with glass containers, and searching for a safer alternative to see-through sippy cups.

So how dangerous are these bottles? And what should consumers do about the risk?

"The truthful answer is that nobody knows" their full health impact yet, said David Ozonoff, a professor of environmental health at the Boston University School of Public Health. "And because we don't know, it's prudent to avoid something that is avoidable."

At the heart of the debate is an odorless, tasteless chemical called bisphenol A that is one of the most commonly used synthetic compounds. It is used to line most canned goods, from soups to soft drinks, to prevent corrosion. It helps make sunglasses and compact discs durable. And it strengthens virtually all transparent, light weight hard plastic baby and water bottles. The chemical has been used for decades, and millions of pounds are produced in the United States each year.

Animal studies have linked exposure to small amounts of bisphenol A to reproductive problems and possible cancers later in life, though the level of risk is unknown. A small body of research suggests that exposure to the chemical in the uterus could contribute to later obesity.

But chemicals that harm animals are not always bad for humans, particularly in the small amounts to which most people are exposed. Industry representatives say no study has proved a link between bisphenol A and health problems in humans.

Still, the US National Toxicology Program, part of the Department of Health and Human Services, last week released a draft report on the chemical, saying there was "some concern for neural and behavioral effects in fetuses, infants, and children at current human exposures." A few days later, Wal-Mart said it would stop selling baby bottles made with bisphenol A by next year and replace them with a bottle free of the chemical. Nalgene, the maker of the durable and ubiquitous hiking bottle whose parent company, Thermo Fisher Scientific Inc., is based in Waltham, also said it would replace its bisphenol A bottles in stores.

Last Friday, Health Canada delivered another blow, announcing that the compound was potentially harmful to people, especially newborns and infants. If no new scientific evidence is brought forward in the next 60 days, officials announced they would ban it from all baby bottles in the country.

"We have concluded that it is better to be safe than sorry," Tony Clement, the Canadian health minister, told reporters.