An air contaminant is commonly classified as a ____.

An air contaminant can be classified in various ways, which makes the answer "All of these" appropriate. Air contaminants can indeed be gases, as many pollutants, like carbon dioxide and ozone, exist in gaseous forms. Additionally, contaminants can also be residues, which may include particulate matter or dust that settles in the air. Lastly, air contaminants can be classified as materials, encompassing a broader category that includes both solid and liquid pollutants that may enter the air due to different activities or processes.

The classification of air contaminants as gases, residues, and materials acknowledges the diverse nature of pollutants present in the air. Understanding this classification helps in recognizing potential health impacts, compliance with regulations, and the implementation of effective air quality management strategies. This comprehensive view is critical for anyone engaged in building maintenance or environmental health and safety, as it underscores the importance of addressing all forms of contaminants to ensure a safe and healthy indoor environment.