Statement 1 is correct. The WHO recommends that the 24-hour mean of PM2.5 should not exceed 15 g/m and the annual mean should not exceed 5 g/m. These are the current most stringent guidelines set by WHO to protect public health.

Statement 2 is incorrect. Ozone pollution typically peaks during sunny and stagnant weather conditions, not during inclement weather like rain or storms.

Statement 3 is incorrect. PM is a common proxy indicator for air pollution. It affects more people than any other pollutant. The major components of PM are sulfate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water. It consists of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air. While particles with a diameter of 10 microns or less, (≤ PM10) can penetrate and lodge deep inside the lungs, the even more health-damaging particles are those with a diameter of 2.5 microns or less, (≤ PM2.5). PM2.5 can penetrate the lung barrier and enter the blood system. Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer.

Statement 4 is correct. Excessive ozone exposure can trigger asthma attacks and worsen respiratory problems. Ozone irritates the respiratory tract, making it harder to breathe.