When incomplete or impurities are added, the combustion process yields gases and particulate matter which pose a potential threat to human health.
Humans are particularly sensitive to particulate matter.
The remainder consists of a wide variety of hydrocarbons and other organic compounds. Combustion is the exothermic process whereby the hydrocarbon is oxidised. Potentially harmful by-products may also result if combustion is inefficient due to an incomplete combustion. Wet wood burns poorly because the pre-ignition phase is prolonged, lowering the combustion temperature. Many of the PICs are strongly associated with chronic respiratory impacts such as increased airway resistance and decreased vital capacity.