The largest U.S. study to date of how day care affects children's health indicates that, while the health risks increase for toddlers, they level off or even go down after age 3.

The explanation for this unexpected finding may be that day care simply shifts the peak age for colds and sniffles, so that children encounter common viruses earlier in life and build up immunity as toddlers rather than when they enter school, said Eugene S. Hurwitz, a medical epidemiologist at the federal Centers for Disease Control and the new study's principal author.

"The data certainly suggest that possibility," he said. "There may actually be some positive aspects to it, though it's unclear whether it's better to get ill early or late."

Respiratory infections are the most common cause of childhood illness, with children under age 5 commonly suffering five or six episodes a year. Sixty-eight percent of American children in that age group are cared for outside their homes during the day, according to the National Center for Health Statistics.

A number of other studies have raised concerns about whether early, frequent exposure to other children has significant medical consequences, such as increased rates of diarrhea, respiratory illness, meningitis and other infections.

But the CDC study, published in this month's issue of the journal Pediatrics, is reassuring on some counts. It suggests, for example, that exposure to viruses in the child care setting probably accounts for only a small percentage of children's colds.

The study was conducted in the spring of 1987. From a random sample of U.S. households, CDC researchers enrolled 2,137 children in the study, 957 of whom were in day care. For the study's purposes, "day care" was defined as being cared for with at least one other unrelated child for 10 or more hours per week.

The children were divided by age into three groups: those younger than 18 months old, those 18 to 35 months old, and those 36 to 59 months old. Parents in the study responded by telephone to a detailed questionnaire assessing their children's health during the preceding two weeks.

The study found that being in day care significantly increased the frequency of colds and respiratory illness only in the youngest group. Children in day care who were less than 18 months old had 60 percent more such infections than children cared for at home.

In the 18- to 35-month-olds, the impact of day care strongly depended upon whether a child had older siblings. For those without older siblings, day care had a major impact. Such children had three times the rate of respiratory infections seen in children with no siblings cared for at home.

But for children in the same age group who had older sisters or brothers, day care did not increase the infection rate, suggesting that such children were already being exposed by their siblings to common viruses and building up immunity.

In the oldest group, day care did not significantly increase the rate of respiratory illness. In fact, children in this age group who had been in day care for more than 26 months were only half as likely to get colds as children of the same age who were cared for at home. Length of day-care experience had a similar impact in the younger groups: Infection rates were lowest in those children who had been in day care longest, suggesting a gradual buildup of immunity.

The findings may mean that a child who has been in day care may actually suffer fewer colds than other children once he reaches elementary school, Hurwitz said. "We haven't confirmed it, but the study certainly suggests that it's possible.".

The data suggested that only about 20 to 30 percent of colds suffered by children attending day care originated in the child care setting. The CDC researchers estimated that about 10 percent of colds in children under 5 can be attributed to day care.

Hurwitz noted that the study took place in the spring and cautioned that the figure could be higher during winter months, the peak season for sniffles.