Last Updated: 2003-05-16 10:00:53 -0400 (Reuters Health)

LONDON (Reuters) - Researchers reporting in a British medical journal said Friday that passive smoking may not be as harmful as previously thought -- leading critics to question the study's method and ties to the tobacco industry.

Inhaling second-hand cigarette smoke has been linked to an increased risk of lung cancer and heart disease, but scientists in New York and California said in a report in the British Medical Journal that the effects may have been overstated.

They based their conclusion on analyzing data from a study funded by the tobacco industry.

"The association between exposure to environmental tobacco smoke and coronary heart disease and lung cancer may be considerably weaker than generally believed," said James Enstrom, of the University of California, Los Angeles, and the study's lead author. He called the backlash against the findings "discouraging."

Some other scientists and the American Cancer Society disputed the findings, saying the research is flawed and inadequate to measure the impact of passive smoking.

"There is overwhelming evidence, built up over decades, that passive smoking causes lung cancer and heart disease, as well as triggering asthma attacks," said Dr. Vivienne Nathanson of the British Medical Association.

"In children, passive smoking increases the risk of pneumonia, bronchitis, and reduces lung growth, as well as both causing and worsening asthma," she added in a statement.

The American Cancer Society's rebuttal was even stronger. "We are appalled that the tobacco industry has succeeded in giving visibility to a study with so many problems it literally failed to get a government grant," said Dr. Michael Thun, the society's national vice president of epidemiology.

The U.S. cancer group said Enstrom's study is based on a small subset of data first collected in 1959, when second-hand smoke was everywhere, and does not distinguish between people who were exposed to smoking and those who were not.

Enstrom said his detractors were applying a double standard because several major studies routinely quoted as proving the risks of second-hand smoke also relied on a comparison between smoking and nonsmoking spouses.

Enstrom and Geoffrey Kabat, of the State University of New York in Stony Brook, analyzed data from a cancer prevention study of 118,094 California adults from 1959 to 1998, which was funded by the tobacco industry.

They focused on more than 35,500 people who had never smoked but who had spouses who did. The researchers found that exposure to passive smoking was not associated with deaths from heart disease or lung cancer.

Enstrom said tobacco industry funding was used toward the end of the study. "The industry has a bad reputation. That doesn't mean they can influence anyone," he said.

Through the 1970s, secondhand smoke was so pervasive that virtually everyone was exposed, whether or not they were married to a smoker, says the American Cancer Society.

Enstrom counters that his study was even more rigorous in keeping track of smoke exposure than major studies routinely quoted as demonstrating the risk of second-hand smoke.

"The study included four assessments -- in 1959, 1965, 1972 and 1999 -- over the 39 year follow-up period," he said.