Breast milk is best, and it can be even better. In addition to immunity-boosting antibodies and potent nutrients, it turns out that breast milk can also contain less desirable ingredients, such as low levels of chemical residues from everyday exposures. The U.S. has already reduced amounts of the most dangerous substances in the environment, and the National Institute of Child Health and Human Development plans to study the long-term effects of chemicals found in breast milk. To keep yours healthy now, Sandra Steingraber, Ph.D., a visiting assistant professor at the Center for the Environment at Cornell University, and author of Having Faith: An Ecologist's Journey to Motherhood (Perseus), recommends taking the following precautions. (Pregnant women can benefit from following this advice as well.)

Drop the dry cleaning.

Have your husband or partner pick up the dry cleaning, remove the plastic, and air the clothes outside or next to an open window. This will reduce your exposure to the suspect solvent perchlor-ethyene. Or find a "wet" cleaner that doesn't use solvents.

Be finicky about fish.

As you may already know, larger fish accumulate more mercury than smaller fish, and certain bodies of water are safer than others. Mercury can damage developing brains, so steer clear of large fish like shark, swordfish, king mackerel, tilefish (and some experts say fresh tuna too), and limit canned tuna and fresh-caught fish to one serving a week. Avoid fish from the Great Lakes or the Hudson River, as they contain higher levels of chemicals known as polychlorinated biphenyls (PCBs).

Splurge for full-serve at the gas station,

have your partner pump, or wear sturdy rubber gloves and walk away while the tank is filling -- gas and its fumes contain the suspect solvents benzene and toluene.

Rethink your renovation plans.

Furniture refinishers and oil-based paint can also contain solvents, so avoid exposure. If you have lead paint on your walls, never have it removed while you are pregnant or breastfeeding. Lead can cause developmental disorders in children.

Limit the use of pesticides.

Avoid chemical exposures by using enclosed bait (such as a "roach motel") rather than a spray insecticide. And don't buy any product made with Dursban or Diazinon; the government is currently phasing these chemicals out of household use. For more information, visit The Natural Resources Defense Council at www.nrdc.org/breastmilk or call 212/727-2700.