Animal data has hinted that nanoparticles can hop the barriers from in the lungs to ride in the bloodstream. But researchers didn't know if that applied to humans. For a new study, researchers recruited 14 healthy male volunteers to breathe in gold nanoparticles of varying sizes for two hours in a chamber where they also did moderate exercise on a stationary bike. The exercise ensured that the participants took deep breaths and got the nanoparticles deep in their lungs.

The gold nanoparticles were similar in size to those found in combustion exhaust, a leading component of air pollution. The finest particulates of air pollution can contain nitrogen oxides, sulfur dioxide, and other toxic ingredients that can induce oxidative stress and inflammation in the body. Gold, on the other hand, is effectively inert.