1) What is diesel exhaust?
Combustion of diesel fuel produces a complex mixture of thousands of different compounds including fine particulate matter (PM2.5), nitrogen oxides (NOx), and other toxic air pollutants. Commonly called soot, diesel pollution accounts for over one quarter of the total hazardous pollution in our air, and 66 percent of particulate pollution is from on-road mobile sources. Soot also contains a significant number of particles, many smaller than 2.5 micrometers, approximately 1/30 the width of a strand of a human hair, which, when inhaled, can lodge deep in lung tissue and potentially cause adverse health effects. 
2) What makes diesel exhaust so dangerous?
The U.S. Environmental Protection Agency (EPA) regards diesel exhaust as one of the greatest public health risks. Exhaust from diesel engines contains many toxics and can be inhaled into the deepest parts of the lungs where it can enter the bloodstream. It is attributable to an increase in premature death, asthma attacks, and emergency-room visits and can exacerbate the symptoms of asthma as well as other chronic obstructive pulmonary diseases, including bronchitis and emphysema.  Scientists and health experts have been unable to identify a safe level of exposure to diesel exhaust, or a level below which exposure has no health effects.  The 2008 State of the Air Report, published by the American Lung Association, stated that there were 151,071 children with asthma, 304,471 adults with asthma, 172,097 persons with chronic bronchitis, and 64,266 persons with emphysema within the Dallas-Fort Worth region, all of which are at risk of having exacerbated health problems due to excessive air pollution. 
In addition to particulate matter and air toxics, the EPA affirms that diesel exhaust is a major contributor to ozone formation due to excess emissions of NOx. Prolonged exposure to ozone can also have adverse health impacts, such as irritation of the airways and reduced lung function. It can also be attributable to coughing, irritation of the throat and chest, and an inability to breathe as deeply or vigorously as normal.
3) Are certain segments of society more vulnerable to diesel exhaust than others?
Yes, diesel exhaust can have severe health implications for the population as a whole; however, children, the elderly, persons that work outside, and residents of urbanized areas are particularly vulnerable to the effects of soot.
Children are more vulnerable than adults because they breathe 50 percent more air per pound of body weight and their respiratory systems are still developing. Children also spend a considerable amount of time outdoors during the summer and the start of the school year (August-October) when ozone levels are typically higher.  Exposure to such pollutants is associated with increased frequency of childhood illnesses and can contribute to the development of asthma, which is the leading serious chronic illness among children and the number one cause of school absenteeism.
Also disproportionately affected by diesel soot are the elderly; particularly those with pre-existing respiratory diseases or other health problems. For sensitive individuals, exposure to very little diesel exhaust can cause a reduction in pulmonary lung function, chest discomfort, coughing, and wheezing.
People most exposed to diesel exhaust are those working outside, particularly near bridges, in tunnels, and at loading docks; truck and bus maintenance garages; toll booths; and construction sites with trucks, forklifts, and material handling machines. In addition, individuals who regularly exercise outdoors are also likely to be exposed to diesel exhaust and high ozone levels.
Soot levels and ozone concentrations are highest in urban environments. Residents living in such areas may be disproportionately exposed to excess exhaust and; thus, more susceptible to potential health problems. The risk of lung cancer from diesel exhaust for people living in urban areas is three times greater than those residing in rural areas.  Pollution levels are also higher near major roadways where people are often exposed to these pollutants during travel, work, and shopping.
 “Health Concerns of Diesel.” State of New Jersey, Department of Environmental Protection. 2007. http://www.stopthesoot.org/dieselhealthconcerns.htm
 Dockery, D.W., et. al., "An Association Between Air Pollution and Mortality in Six U.S. Cities," New England Journal of Medicine, 329:1753-59, 1993. Pope III, C.A., et al., "Particulate Air Pollution as a Prospective Study of U.S. Adults," American Journal of Respiratory Critical Care Medicine, 151:669-674, 1995; Shprentz, D., "Breathtaking: Premature Mortality Due to Particulate Air Pollution in 239 American Cities," New York, Natural Resources Defense Council, May 1996, pp. 13-32
 Pope, C.A., et. al., "Lung Cancer, Cardiopulmonary Mortality, and Long-term Exposure to Fine Particulate Air Pollution", Journal of the American Medical Association, March 6, 2002. (each 10 microgram/cubic meter increase in PM leads to 8 percent increased risk of lung cancer deaths, a 6 percent increased risk of cardiopulmonary mortality/heart attacks, and a 4 percent increased risk of death from general causes)
 American Lung Association: State of the Air 2008. www.lungfla.org/Portals/63/SOTA2008.pdf. Population data based on nine-county DFW 8-hour ozone nonattainment area.
 TCEQ – DFW SIP pp. 1-4
 Schneider, C.G., Hill, L.B, PhD. “Diesel and Health in America: The Lingering Threat.” Clean Air Task Force. February