In air quality control, what does "purity" refer to?

Purity in air quality control specifically refers to the presence of pollutants within the air. This concept is crucial in assessing the air's overall quality and safety for human health and the environment. When we talk about air purity, we are primarily concerned with the concentration and types of contaminants that may be present, such as particulate matter, volatile organic compounds (VOCs), and other harmful substances. High purity means that the air has a low level of these pollutants, indicating a cleaner and healthier atmosphere.

Conversely, when considering the other options, their relevance to air quality is different. The color of the air is not a scientific measure of air quality; rather, it might pertain to visibility issues rather than pollutants. Temperature of the air can affect comfort levels but does not directly indicate the air's cleanliness or pollution levels. Humidity levels, while important for comfort and certain health aspects, do not directly correlate with the presence of pollutants and thus are not indicative of air purity. Therefore, the emphasis on pollutants when defining purity underscores its critical role in air quality assessments.