Carbon monoxide is a highly toxic air pollutant that can kill you, but it might also have a calming effect on the jittery nerves of overstressed city dwellers.
That's the unexpected conclusion reached by Itzhak Schnell of Tel Aviv University, who claims to have discovered that urbanites who breathe in extremely low levels of the gas, in the range of 1-15 parts per million, are innoculated against the stressful effects of crowds and noise. (It should be noted that his study was small and by all accounts preliminary -- so verification of this result will require, as in all matters scientific, multiple follow-up studies.)
Carbon monoxide is the most common cause of poisoning in the U.S., so it's important to emphasize that the levels of ambient CO found by Professor Schnell were actually 1/10th what he expected to find when he embarked on this study, low enough to not be a concern, health-wise.
Schnell originally set out to measure what he calls "environmental load," which is the cumulative burden of stressors urbanites experience. He looked at stress from temperature changes, crowds, carbon monoxide and noise. In addition to suggesting that tiny amounts of carbon monoxide actually reduce this load, his work shows that the easiest way to reduce it is to eliminate noise.
Most of the noise in cities, he notes, is due to human activities -- so being a good neighbor is just as important as investing in sound-proof windows.