In the context of WHO Air Quality Guidelines, consider the following statements : 1. The 24-hour mean of PM2.5 should not exceed 15 µg/m³ and annual mean of PM2.5 should not exceed 5 µg/m³. 2. In a year, the highest levels of ozone pollution occur during the periods of inclement weather. 3. PM10 can penetrate the lung barrier and enter the bloodstream. 4. Excessive ozone in the air can trigger asthma. Which of the statements given above are correct ?

Statement 1 is correct as per the updated 2021 WHO Air Quality Guidelines, and Statement 4 is a medically established fact regarding ozone’s role as a respiratory irritant. Statement 3 is the most tempting error; it is incorrect because while PM10 can reach the lungs, only PM2.5 (and smaller ultrafine particles) are small enough to penetrate the lung-blood barrier and enter the bloodstream. The core concept being tested is the specific physiological impacts and updated concentration thresholds of major air pollutants defined by the World Health Organization.