CHICAGO (Reuters) - About half the early childhood cases of asthma, chronic bronchitis and wheezing are attributable to exposure to second-hand cigarette smoke, U.S. researchers said on Monday. In the United States, researchers extrapolated results from the study of more than 7,000 children between the age of two months and 2 years to estimate that environmental tobacco smoke (ETS) caused an extra 160,000 cases of asthma per year, 79,000 cases of bronchitis and 172,000 cases of wheezing. Based on data compiled between 1988 and 1994 that included reports on household smoking and maternal smoking during pregnancy, the researchers estimated that second-hand smoke was responsible for between 40 percent and 60 percent of cases of asthma, bronchitis and wheezing among young children. "These findings reinforce the need to reduce the exposure of young children to ETS," study author Peter Gergen of the Center for Primary Care at the Agency for Health Care Policy and Research in Rockville, Maryland, wrote. Several other government health agencies participated in the study. The study, which appeared in the journal Pediatrics, published by the American Academy of Pediatrics, found separately that 38 percent of children breathed in cigarette smoke from others' smoking at home, and 24 percent of children were exposed in the womb from their mothers smoking.