Question

how does poor indoor air quality most commonly affect human health? by increasing outdoor air pollution by causing respiratory problems by reducing air circulation by improving energy efficiency

Explanation:

• "by increasing outdoor air pollution": Poor indoor air quality doesn't directly increase outdoor pollution; this is incorrect.
• "by causing respiratory problems": Poor indoor air (with pollutants, allergens, etc.) is known to irritate the respiratory system and cause issues like asthma, bronchitis, etc., so this is correct.
• "by reducing air circulation": Poor air quality is a result of reduced circulation in some cases, but it's not how it affects health; this is a cause or related factor, not the health effect.
• "by improving energy efficiency": Energy efficiency is unrelated to health impacts of air quality.

Answer:

by causing respiratory problems