The City of Chicago is retrofitting a portion of the City's diesel-powered fleet using oxidation catalysts (catalytic converters). Diesel exhaust contains fine particles that pass through the nose and throat, which can cause lung damage and other significant health problems and aggravate respiratory conditions such as asthma and bronchitis. It also contributes to haze and ozone formation (a component of smog), acid rain and global climate change.

Oxidation catalysts chemically convert harmful hydrocarbons and carbon monoxide into water vapor and carbon dioxide. In the process, particulate matter emissions are reduced by 20-30%, hydrocarbons by 50% and carbon monoxide by 40%.

The City plans to retrofit the entire diesel fleet over time, pending funding availability. The retrofits will improve air quality while reducing the risk of health problems related to diesel exhaust. The project will also act as a model for other fleets to follow.