What did most to keep hydroxyl's profile low, however, was the apparent harmlessness of CO - the pollutant that consumed half of all the hydroxyl in the atmosphere. Emitted mostly from forest fires and fossil fuels, CO had for many years been the Cinderella pollutant. Its presence in the atmosphere did not seem troublesome, even though concentrations had tripled between 1800 and 2000 and carried on rising. It wasn't very long-lived. It didn't cause acid rain or global warming. It simply didn't show up on any major environmental agenda