Senior author Dr. Ronald Crystal, chief of the division of pulmonary and critical care medicine at NewYork-Presbyterian/Weill Cornell, says genes commonly activated in the cells of heavy smokers are also turned on or off after very low-level exposures.
"Even at the lowest detectable levels of exposure, we found direct effects on the functioning of genes within the cells lining the airways," Crystal says in a statement.
Crystal and colleagues tested 121 people who were non-smokers, active smokers and low exposure smokers for urine levels of nicotine and cotinine -- markers of cigarette smoking within the body.
The researchers scanned each study participant's entire genome to determine which genes were either activated or deactivated in cells lining the airways.
The study, published in the American Journal of Respiratory and Critical Care Medicine, finds there was no level of nicotine or cotinine that did not also correlate with genetic abnormalities.
"This means that no level of smoking, or exposure to secondhand smoke, is safe," Crystal says.
Notable deaths of 2014 [PHOTOS]