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. Most researchers believe that these impacts far outweigh all other possible human health impacts. As humans are acutely sensitive to fine particulate compared to other contaminants, the inhalable particulate portion of the PIC is an appropriate one on which to focus combustion-related air quality impacts and management strategies.