Many parents worry that their children may be harmed by exposure to environmental factors they cannot avoid or control, including pesticide residues on fruits and vegetables, approved food additives, chlorinated drinking water and hormones in milk.

They fear electromagnetic fields as a cause of childhood leukemia, a mercury preservative in vaccines as a cause of autism, and alar, a growth stimulant on apples, as a cause of cancer.

None of these are actual hazards. But even if they were, they are hardly the main hreats to the health and lives of fetuses, infants, children and adolescents, says Dr. Robert L. Brent, a professor at Thomas Jefferson University in Philadelphia and a leading expert on what is and is not known about the effects of environmental chemicals and physical agents on developing humans.

In the concluding chapter of a recent report, published as a supplement in the journal Pediatrics, Dr. Brent and his co-author, Dr. Michael Weitzman, a pediatrician at the University of Rochester School of Medicine and Dentistry, reviewed what are unquestionably the leading risks to infants, children and adolescents.

Most of the hazards that take the greatest toll on the health and lives of the young people in this country can be prevented, without any need for further research, legislation, environmental cleanup or any other measure that requires the action of anyone besides parents and caretakers. "Nearly every day a child in Florida dies in a swimming pool," Dr. Brent said. "No environmental agent exacts such a toll."

Accidents are the leading cause of death in children under 15. But while the word accident implies an unexpected and unavoidable event, most accidents involving children could be prevented by vigilance. Here are the most important hazards.

Sudden infant death syndrome Risk is reduced by putting infants to sleep on their backs and providing a nonsmoking environment.

Falls Infants can suffer head injuries falling from strollers, down stairs, off beds or against sharp-pointed furniture. Toddlers and children aged 5 to 9 fall from windows, stairs, trees, garage roofs and ladders.

Vehicular accidents Infants and children under 10 should never ride in the front seat, and those under 80 pounds should always ride in a properly installed car seat or booster seat appropriate to the child's age and size.

Outside the car, children under 10 are at risk of death from pedestrian accidents, including being run over by the family car in their own driveway and ignoring safety rules when crossing the street. Their small size makes them hard for drivers to see.

For teenagers, reckless driving, impulsive behavior and drunken driving make auto accidents a leading cause of death.

Burns Infants can suffer burns from kitchen equipment or hot items pulled off the table, as well as hot water in a tub and uncovered radiators. Toddlers should never have access to matches, cigarette lighters or fuel-filled or flint igniters.

House fires can be started by adults who fall asleep while smoking, faulty wiring, defective heating systems and space heaters.

Every home should be equipped with one or more working smoke alarms.

Poisoning As soon as children can crawl, they are at risk of poisoning from medications, household chemicals (including drain and oven cleaners, alcohol and paint thinner), pesticides and rodent killers. Such items should be stored out of children's reach in cupboards with childproof locks.

Lead poisoning, though much reduced, is still a risk for millions of children who live in old homes with lead-based paint, plaster or putty, as well as those with old toys, cribs and imported pottery.

A child's blood lead level should be checked and elevated levels treated to prevent cognitive deficits.

Drowning "Pools, hot tubs and wading pools must have supervision and be fenced in, locked or covered," the Pediatrics authors wrote.

The incidence of childhood drownings varies by state according to the number of backyard swimming pools and hot tubs, but is also a problem in bathtubs, bathinettes and kiddie wading pools. An infant or young child in or around water should never be left unattended, not even for a minute. Children relying on flotation devices and those under 4 who can swim are not safe in the water. Children should always swim with another swimmer.

Impetuous, risk-taking teens are also at risk of drowning. When boating, every infant and child - even good swimmers - should wear an approved life jacket.