Which of the following is NOT considered a contaminant suspended in the air?

The option that is not considered a contaminant suspended in the air is gases like nitrogen. In the context of air quality and pollution, contaminants typically refer to any substances that can be harmful or undesirable when present in the atmosphere.

Solid particles, whether they are visible or microscopic, are common forms of air contaminants. These can include dust, pollen, soot, and other particulate matter. Living organisms, such as bacteria or pollen from plants, can also be categorized as contaminants because they can affect air quality and pose health risks.

On the other hand, nitrogen is a naturally occurring gas that constitutes a significant portion of the Earth's atmosphere—about 78% of the air we breathe. It is not a contaminant in this context because it does not have adverse effects on health or the environment when found in its typical concentrations. Thus, gases like nitrogen are not classified as contaminants suspended in the air as they are part of the normal composition of the atmosphere.