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 PM_2.5, 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?

The correct answer is 1 and 4 only.

Let us analyze each statement step-by-step to understand why statements 1 and 4 are correct, while statements 2 and 3 are incorrect:

Statement 1 is correct:
According to the World Health Organization (WHO) Global Air Quality Guidelines (updated in 2021), the recommended guideline level for particulate matter PM_2.5 is an annual mean of 5 ug/m³ and a 24-hour mean of 15 ug/m³. These levels are designed to protect public health from the adverse effects of fine particulate matter exposure.

Statement 2 is incorrect:
Ground-level ozone (O₃) is a secondary pollutant formed by photochemical reactions between oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) in the presence of sunlight and heat. Consequently, ozone levels are highest during warm, sunny, dry, and stagnant summer weather conditions, rather than during inclement (cloudy, rainy, or wet) weather.

Statement 3 is incorrect:
Particulate matter size determines how deep the particles can penetrate the respiratory tract. PM₁₀ (particles with a diameter of 10 micrometers or less) can penetrate deep inside the lungs, but they are generally too large to easily cross the lung-blood barrier. It is primarily PM_2.5 (fine particles) and even smaller ultrafine particles (PM_0.1) that are capable of penetrating the lung barrier and entering the bloodstream directly.

Statement 4 is correct:
Ozone is a powerful respiratory irritant. High concentrations of ozone in the air can irritate the airways, cause breathing difficulties, reduce lung function, and trigger symptoms in people with pre-existing chronic respiratory illnesses, notably asthma.

Therefore, only statements 1 and 4 are correct.