It's part of Roberts' eight years of work on bisphenol A, an ingredient in plastics ranging from reusable food containers to eyeglass lenses to CDs.

It's also part of her life as a mom: Many baby bottles contain BPA.

"I wholeheartedly believe there are serious concerns with this compound," she says, thoughtfully fingering a test tube partly filled with the white, powdery substance.

Others agree. Two weeks ago, a draft report by a program of the National Institutes of Health concurred with the earlier evaluation of an independent scientific panel, concluding there was "some concern" about possible neural and behavioral effects in fetuses, infants and young children exposed to the chemical at current levels.

The plastics industry says products containing BPA are safe.

Nevertheless, some stores quickly began pulling baby bottles with BPA from their shelves. Manufacturers are working to eliminate the substance. Legislators have proposed bans.

Roberts, in addition to her research, cowrote an essay - "Babies, Bottles and Bisphenol A: The story of a Scientist-Mother" - that PLoS Biology, a journal of the Public Library of Science, published last summer.

A five-minute walk across campus leads to Roberts' backyard, where Siena Johnson, 21/2, puts down her plastic sippy cup and leaps up from a plastic picnic table, exclaiming happily, "My mommy's here!"

One-year-old Bristow Johnson is fretful, so in the kitchen, where plastic cups and lids form a colorful pile in the dish drain, Roberts fixes a plastic bottle of formula.

In the living room, Siena drops to the floor to play with her plastic dolls.

"That's one of the problems," says Roberts. "As a mother, you'd go insane if you tried to take away every single plastic thing from your child. That's all there is."

For that matter, she doesn't want to take away all plastics. They're washable and bleachable, have no splinters, and the embedded colorant won't chip off.

So she prioritizes, focusing on clear, hard plastics (more likely to contain BPA) that will probably end up in her children's mouths.

Bristow's bottles are made by Medela, one of the BPA-free brands that have proliferated in just the short time since Siena was an infant.

Siena's heart was set on a Dora the Explorer sippy cup. Roberts was relieved to discover it, too, was BPA-free.

Bisphenol A is a chemical building block that makes polycarbonate plastic tough, lightweight, shatter-resistant and clear.

It's in helmets, goggles and dental sealants. It's also used in medical devices such as dialyzers and incubators - like the one that Bristow, born prematurely, stayed in for his first 69 days.

And BPA is in the epoxy linings for virtually all canned goods, protecting the food from a metal that might corrode or affect flavor.

But BPA can migrate from these substances to humans - mostly through food or drink. It has been found in human blood, urine and breast milk.

A 2003-04 study by the U.S. Centers for Discase Control and Prevention found detectable levels of BPA in 93 percent of 2,517 urine samples from people aged 6 or older.

Concern arose after reproductive and developmental effects were reported in laboratory animals. BPA is considered to be "weakly" estrogenic, so researchers are investigating its effect on infant development and hormone-related diseases like breast and prostate cancer.

Within days of the NIH's recent National Toxicology Program report, the Canadian government, based on its own risk assessment, began moving toward a ban of baby bottles with BPA.

Wal-Mart, among other stores, has begun pulling baby bottles with BPA from the shelves; a spokesman said the company expects all of its baby bottles to be BPA-free early next year.

Playtex Infant Care is distributing one million free no-BPA "Playtex Drop-Ins Original Nurser Systems" bottle liners. "While U.S. and worldwide regulatory bodies continue to deem the ingredient safe," the company says, "we are listening to consumer concerns."

Following California, New Jersey legislators have introduced bills banning BPA in toys and child-care products.

The chemical industry has cricticized many of these moves.

"Although I'm sure their intention is to do things that are good for their customers, they're not going to improve health or safety of their customers by taking these products off the shelves," says Steven Hentges, executive director of the American Chemistry Council's Polycarbonate/BPA Global Group.

"We believe BPA is safe for use, based on many scientific reviews," says Hentges, who was authorized to speak for manufacturers of BPA.

There's also the matter of replacing it. "If we want to not use BPA, we will not have polycarbonate plastics. That becomes a real big challenge," he says. "You will find no alternatives that have been tested so well as bisphenol A."

Meanwhile, the research continues.

The National Institute of Environmental Health Sciences, one of the major funders of BPA studies, has given Roberts a three-year, $150,000 grant to study its effect in a new area, the immune system.

"So from that standpoint, it's a unique grant that's very valuable," says Jerrold Heindel, an institute program official. "Her results will be important to help us determine if there should be concerns."

In her Collegeville lab, Roberts and her research students are looking at cathepsins - enzymes that act like small scissors, cutting to pieces whatever is brought into a body's infection-fighting white blood cell. Some pieces return to the cell surface and become potential flags for the immune system.

Basically, if the cathepsins do their job differently than they are supposed to, the immune response may be faulty.

So Roberts and her students have been injecting Cheerios-shaped cereal with BPA and feeding it to mice. Later, they collect and analyze the mice's white blood cells.

While results are preliminary, she believes she's seeing problems.

Roberts is applying for more grants to continue her enzyme work. She also hopes to start an outreach program in the area, educating mothers about BPA and how to avoid it.

"Do I think more science needs to be done? Always," the biologist says.

Adds Roberts the mother: "I have two little kids at home. I want to make sure they're healthy and growing up the best they can."

How to Limit Your Exposure

Consumers eager to avoid suspect plastics won't find the going easy. Labeling is not always required for all ingredients. But toxicology experts say taking the following steps can lower possible risk.

- Ranit Mishori, Washington Post

For suspect plastics in general

Avoid placing hot food or liquids in plastic containers. Use glass, ceramic or stainless-steel containers instead. Heating plastics to high temperatures promotes the leaching of chemicals out of containers and into the food or liquid they hold. (Freezing liquids in plastic bottles poses no such risk.)

When heating in a microwave, use only cookware labeled "microwave safe." (Remove food from plastic wrap before thawing or heating in a microwave.)

Buy products in cardboard cartons instead of plastic containers.

Check recycling codes for clues about plastics components you may want to avoid: Recycling code 7 may mean the product contains bisphenol A. Recycling code 3 may indicate DEHA, which belongs to a separate group of chemicals, known as phthalates, that have also raised concerns.

For bisphenol A

Look for "BPA-free" claims on toys, baby bottles and containers. There's been a recent explosion of such products (many priced higher than standard versions).

Reduce use of canned food. Eat fresh or frozen foods. Bisphenol A has been found in the lining of canned food tins.

Avoid polycarbonate and PVC (polyvinyl chloride) plastics, both of which contain BPA. Alternatives include polyethylene plastic (also labeled PETE) and containers marked with recycling code 1, 2 (HDPE) and 4 (LDPE). Polypropylene (recycling code 5, or PP) is also safe.

If you use hard polycarbonate plastics (Nalgene bottles, baby bottles, sippy cups), do not heat or use them for warm or hot liquids.

Do not wash polycarbonate plastic containers in the dishwasher with harsh detergents.

For phthalates

A group of compounds called phthalates has raised concerns similar to those involving BPA.

Look for phthalate-free toys or those approved by the European Union (EU).

Plastic wraps generally should not be heated or placed in a microwave oven. Those that are labeled microwave-safe should be placed loosely over containers. Make sure they do not touch the food.

Avoid synthetic fragrance in personal-care products. While the Food and Drug Administration requires the listing of ingredients on cosmetics sold in retail stores, it does not require the listing of individual fragrance ingredients.

Check nail polish labels. The FDA requires phthalates be listed unless they are a fragrance ingredient. (Some nail polishes contain them to reduce cracking.)

SOURCES: National Institute of Environmental Health Sciences; Institute for Agricultural and Trade Policy; National Geographic Green Guide

Links to additional articles, resources, the recent federal report that raised safety concerns, and Rebecca Roberts' essay: http://go.philly.com/plastics