Human Health

Diesel exhaust contains significant levels of small particles, known as fine particulate matter. Fine particles are so small that several thousand of them could fit on the period at the end of this sentence.
Fine particles pose a significant health risk because they can pass through the nose and throat and lodge themselves in the lungs. These fine particles can cause lung damage and premature death. They can also aggravate conditions such as asthma and bronchitis.
Nationwide, particulate matter, especially fine particles, is responsible for thousands of premature deaths every year.
EPA has determined that diesel exhaust is a likely human carcinogen. Diesel exhaust can also contribute to other acute and chronic health effects (see EPA's Health Assessment Document for Diesel Exhaust).

People with existing heart or lung disease, asthma or other respiratory problems are most sensitive to the health effects of fine particles. The elderly and children are also at risk.
Children are more susceptible to air pollution than healthy adults because their respiratory systems are still developing and they have a faster breathing rate.

Diesel exhaust also contains pollutants that contribute to ozone formation (or smog), acid rain, and global climate change.
Fine particles from diesel engines contribute to haze which restricts our ability to see long distances.