DDT in human milk. What determines the levels?

In a study of fifty-five human milk samples from eastern Pennsylvania, the total DDT derived products exceeded the DDT content of cows milk and canned infant formula. Total DDT in human milk exceeded the U.S. Food and Drug Administration's tolerance for total DDT in cows milk in 47% of the samples. Four factors were identified statistically as accounting for 54% of the variance on total DDT levels in human milk. These factors are: (1) number of children nursed; (2) number of cigarettes smoked per day; (3) use of non-persistent pesticides; and (4) diet in calories.