In some cases, the airborne particles have been shown to increase the incidence of disease occurrence, and in others to exacerbate the symptoms of pre-existing diseases. There is convincing evidence that exposure to high concentrations of some types of PM2.5 particulate materials, for example cigarette smoke and diesel exhaust, can increase the risk and severity of both acute and chronic respiratory conditions, and studies also indicate that acutely high PM2.5 concentrations can exacerbate the symptoms of pre-existing lung conditions such as asthma.