Is formaldehyde a primary or secondary pollutant?

Formaldehyde is considered a primary pollutant because it is directly emitted into the atmosphere from sources such as vehicle exhaust, industrial processes, and the use of certain household products. Unlike secondary pollutants, which form in the atmosphere through chemical reactions between primary pollutants, formaldehyde is released in its original form. Its presence can lead to various health and environmental concerns, particularly in indoor air quality.