The threat comes from the chemical isocyanic acid (HNCO), which dissolves into the moist tissues of the body and promotes inflammation.
It is released into the atmosphere by forest fires and woodburning, and is also present at high levels in cigarette smoke.
But currently isocyanic acid, which is difficult to detect, is not listed as a "harmful" or "potentially harmful" element of wood smoke or tobacco products.
Scientists in the US used a specially designed mass spectrometer - a machine that carries out chemical analysis by using a magnetic field to deflect atoms - to study exposure levels of isocyanic acid.
Lead researcher Dr Jim Roberts, from the National Oceanic and Atmospheric Administration in Boulder, Colorado, said: "We found isocyanic acid in a number of places, from air in downtown Los Angeles and air downwind of a Colorado wildfire, to cigarette smoke.
"We also demonstrated that it dissolves readily in water, which means that humans can be exposed directly if it gets into eyes or lungs."