Carbon monoxide (CO), a tasteless, colourless and odourless gas emitted by vehicle exhausts and generators, dubbed the silent killer, can also soothe frayed nerves. Itzhak Schnell, professor of geography and human environment at Tel Aviv University, discovered how low CO levels have a calming effect on urbanites caught up in a noisy and stressful environment. They asked 36 healthy volunteers aged between 20 and 40 years to spend two days in Tel Aviv, Israel's busiest city, the journal Environmental Monitoring and Assessment reports. They took various routes to busy streets, restaurants, malls and markets, by public and private transportation or by foot, according to a Tel Aviv statement. Researchers monitored the impact of four stressors: thermal load (heat and cold), noise pollution, CO levels, and the impact of crowds. Noise pollution emerged as the most significant cause of stress. The most surprising finding, said Schnell, was looking at levels of CO that the participants inhaled during their time in the city. Not only were the levels much lower than the researchers predicted - approximately one to 15 parts per million every half hour - but its presence appeared to have a narcotic effect on participants, counteracting the stress caused by noise and crowd density. Though participants exhibited rising stress levels throughout the day, CO had a mitigating influence and extended exposure to the chemical had no lasting effects. "We would be able to tell more accurately under what conditions vulnerable people shouldn't go out, and more importantly, identify areas that are still safe, helping to increase freedom of movement," noted Schnell.