Tobacco companies knew that cigarette smoke contained radioactive alpha particles for more than four decades and developed “deep and intimate” knowledge of these particles’ cancer-causing potential, but they deliberately kept their findings from the public, according to a new study by UCLA researchers.

The analysis of dozens of previously unexamined internal tobacco industry documents, made available in 1998 as the result of a legal settlement, reveals that the industry was aware of cigarette radioactivity some five years earlier than previously thought and that tobacco companies, concerned about the potential lung cancer risk, began in-depth investigations into the possible effects of radioactivity on smokers as early as the 1960s.

“The documents show that the industry was well aware of the presence of a radioactive substance in tobacco as early as 1959,” the authors write. “Furthermore, the industry was not only cognizant of the potential ‘cancerous growth’ in the lungs of regular smokers, but also did quantitative radiobiological calculations to estimate the long-term lung radiation absorption dose of ionizing alpha particles emitted from cigarette smoke.”

The study, published online Sept. 27 in Nicotine & Tobacco Research, the peer-reviewed journal of the Society for Research on Nicotine and Tobacco, adds to a growing body of research detailing the industry’s knowledge of cigarette smoke radioactivity and its efforts to suppress that information.

“They knew that the cigarette smoke was radioactive way back then and that it could potentially result in cancer, and they deliberately kept that information under wraps,” said the study’s first author, Hrayr S. Karagueuzian, an adjunct professor of cardiology who conducts research at UCLA’s Cardiovascular Research Laboratory, part of the David Geffen School of Medicine at UCLA. “Specifically, we show here that the industry used misleading statements to obfuscate the hazard of ionizing alpha particles to the lungs of smokers and, more importantly, banned any and all publication on tobacco smoke radioactivity.”

The radioactive substance — which the UCLA study shows was first brought to the attention of the tobacco industry in 1959 — was identified in 1964 as the isotope polonium-210, which emits carcinogenic alpha radiation. Polonium-210 can be