Which of the following is a major primary pollutant?

• A. Oxygen
• B. Ozone
• C. Mercury
• D. Lead

Answer: (C)

Primary pollutants are substances that are released directly into the air from a source and remain in a form that can affect air quality. Ozone is not emitted as a pollutant itself; it forms in the atmosphere when NOx and VOCs react in sunlight, so it’s a secondary pollutant. Oxygen, while essential for life, is not considered a pollutant at normal levels. Mercury fits the idea of a major primary pollutant because it is directly emitted into the air from sources like coal-fired power plants, mining, and waste incineration, and it is highly toxic, persistent, and bioaccumulative in ecosystems. This direct release and its severe health and environmental impacts make it stand out among the options. (Lead is also a primary pollutant, but mercury is commonly highlighted as a major one due to its widespread toxicity and long-range effects.)