Asthma. New medical research shows that auto-caused air pollution can set the stage for childhood asthma while the helpless fetus is still in the womb. The research indicates air pollution causes genetic alterations before the baby's born, making the infant more prone to asthma. The trigger for environmentally-caused asthma are chemicals called polycyclic aromatic hydrocarbons (PAHs). PAHs are byproducts of incomplete combustion from carbon-containing fuels, resulting in high levels in heavy-traffic areas. Exposure to PAHs has been linked to cancer.