Carbon Monoxide (CO)

Detailed Explanation

Carbon monoxide is a toxic gas that forms when carbon-based fuels — gasoline, natural gas, wood, coal — burn incompletely. Motor vehicles are the largest outdoor source, though industrial processes and residential heating also contribute. CO is dangerous because it binds to hemoglobin in the blood far more readily than oxygen does, reducing the blood's ability to carry oxygen to organs and tissues. At low concentrations, CO exposure causes headaches, dizziness, and fatigue. At high concentrations, it can cause confusion, loss of consciousness, and death. Outdoor CO levels in US cities have decreased substantially since the 1970s due to catalytic converters in vehicles and cleaner-burning engines. The EPA's 8-hour standard for CO is 9 parts per million. Today, CO is almost never the dominant outdoor pollutant in US cities — it appears as the primary pollutant on less than 1% of monitored days nationally. However, CO remains a serious indoor air quality concern, particularly from malfunctioning furnaces, generators, and gas stoves in poorly ventilated spaces. AirHistory includes CO in pollutant day breakdowns for each city, though cities where CO dominates are exceptionally rare in the EPA's modern monitoring data.