Odor pollution is simply the unpleasant stench of such things as garbage, sewage, chemicals, rotting organic matter, and hazardous waste. Human beings can detect foul odors at concentrations as low as one part per trillion, depending on what the chemical is. This is important because people can actually detect air pollutants (except for odorless ones such as carbon monoxide) at levels much lower than can be detected by instruments. This is especially true of the smellier pollutants, such as hydrogen sulfide (H2S), which produces that familiar rotten egg smell, even when diluted in water. Hydrogen sulfide pollution can cause everything from eye and throat irritation to asthma attacks and even death.