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?

This question tests your knowledge of WHO Air Quality Guidelines and air pollutants.

Statement 1 (Correct): According to the WHO guidelines, the annual average for PM2.5 should not exceed 5 µg/m³, and the 24-hour average should not exceed 15 µg/m³.

Statement 2 (Incorrect): Ground-level ozone, a major component of photochemical smog, is typically highest during periods of sunny, warm weather, not inclement weather. Inclement weather usually reduces ozone formation.

Statement 3 (Incorrect): While PM10 (particles with a diameter of 10 microns or less) can penetrate deep into the lungs, it's PM2.5 (particles with a diameter of 2.5 microns or less) that poses a greater health risk as they can penetrate the lung barrier and enter the bloodstream.

Statement 4 (Correct): Excessive ozone in the air is a known respiratory irritant and can indeed trigger asthma and other respiratory problems.

Statement 1 is correct. Statement 2 is incorrect. Statement 3 is incorrect. Statement 4 is correct.

Hence, only statements 1 and 4 are correct.