Which airborne material is least likely to be affected by air filters or indoor air handling equipment?

The correct answer is carbon dioxide. This gas is a naturally occurring component of the air and is a byproduct of respiration, combustion, and various industrial processes. Unlike particulate matter, which consists of solid or liquid particles suspended in the air and can be trapped by filters, carbon dioxide is a gas and passes through air filters without significant obstruction.

Airborne materials like particulate matter can be effectively reduced by air handling systems designed to capture these larger particles. Water vapor, as a gaseous form of moisture, also doesn't get removed by standard air filters. In contrast, carbon monoxide is a gas produced during incomplete combustion and can be detected and managed by certain ventilation systems, although it can be more readily removed from the air compared to carbon dioxide.

Thus, carbon dioxide's molecular structure and behavior as a gas make it least likely to be affected by air filters or indoor air handling equipment.