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?

Answer: B. Statements 1 and 4 only are correct. WHO updated its Global Air Quality Guidelines in September 2021 with stricter standards. Statement 1 is CORRECT. WHO's 2021 guideline for PM2.5: 24-hour mean should not exceed 15 micrograms per cubic metre; annual mean should not exceed 5 micrograms per cubic metre. These are the new tighter standards (previously 25 and 10 respectively). Statement 2 is WRONG. The highest ozone pollution levels occur during HOT, SUNNY, STAGNANT weather conditions — not 'inclement' (bad/stormy) weather. Ozone formation requires sunlight (UV-driven photochemistry of NOx and VOCs) and heat. Inclement weather (rain, storms, low temperatures) actually reduces ozone formation. Statement 3 is WRONG. PM10 particles are larger than PM2.5 and are typically captured in the upper respiratory tract (nose, throat, large airways). PM2.5 (smaller) can penetrate deep into the lungs AND enter the BLOODSTREAM. The statement confuses PM10 with PM2.5. Statement 4 is CORRECT. Excessive ground-level OZONE can trigger ASTHMA, reduce lung function, and worsen respiratory diseases. WHO and CDC list ozone as a major asthma trigger. Source: WHO Global Air Quality Guidelines 2021 / EPA AirNow.