During the combustion of natural gas, if there is insufficient oxygen supply or temperature, complete combustion cannot occur. As a result, some of the methane (CH4) present in natural gas may be partially oxidized to produce formaldehyde (HCHO), which is an aldehyde. Formaldehyde has a distinctive pungent odor and is a volatile organic compound (VOC) that can contribute to indoor air pollution. It is also classified as a carcinogen by the International Agency for Research on Cancer (IARC).

Therefore, it is important to ensure that natural gas appliances are properly maintained and operated with adequate ventilation to promote complete combustion and minimize the production of harmful pollutants like formaldehyde.