What is a major source of indoor air pollution?

Indoor air pollution can stem from various sources, and all of the options listed contribute significantly to the overall problem. Cigarette smoke is a well-known source, releasing numerous harmful chemicals that not only impact the smoker but also affect anyone exposed to the secondhand smoke. Natural gas stoves can emit particulates and gases such as nitrogen dioxide and carbon monoxide, which are harmful when inhaled over time and can lead to respiratory issues. Household cleaning products often contain volatile organic compounds (VOCs) that can evaporate at room temperature, releasing pollutants into the air that can cause both short-term and long-term health effects.

Given that each of these options is a valid contributor to indoor air pollution, it is accurate to state that a comprehensive understanding of the topic recognizes that multiple sources exist simultaneously, thereby making "all of the above" the correct answer. Addressing indoor air quality requires awareness of all these sources and their potential impact on health.