Particulate matter can take many forms, including ash and dust,and motor vehicle exhaust is a major source.
The smallest particles are widely believed to cause the most damage since they are inhaled deep into the lungs and can possibly enter the bloodstream (Environmental Protection Agency, 2003).
The mechanisms through which particles harm health are controversial, with a leading theory being that they cause an inflammatory response that weakens the immune system (Seaton et al., 1995).
Since particles cannot cross the placenta, they would have to damage the fetus indirectly by provoking inflammation in the mother.