In the context of WHO Air Quality Guidelines, consider the following statements: 1. The 24-hour mean of PM 2.5 should not exceed 15 ug/m3 and annual mean of PM2.5 should not exceed 5 ug/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?

1. Correct. WHO Air Quality Guidelines (2021) prescribe PM2.5 annual mean = 5 ug/m3 and 24-hour mean = 15 ug/m3.
2. Incorrect. Ground-level ozone usually peaks in sunny, warm, stable weather due to photochemical reactions, not during inclement weather.
3. Incorrect. It is PM2.5 (and finer particles) that can penetrate deep into lungs and enter the bloodstream; PM10 is generally larger and mainly deposits in the respiratory tract.
4. Correct. Excess ozone exposure can irritate airways and trigger/worsen asthma.

Hence, statements 1 and 4 only are correct.