In the context of WHO Air Quality Guidelines, consider the following statements : The 24-hour mean of PM$_{2.5}$ should not exceed 15 $\mu$g/m$^3$ and annual mean of PM$_{2.5}$ should not exceed 5 $ \mu$g/m$^3$. In a year, the highest levels of ozone pollution occur during the periods of inclement weather. PM$_{10}$ can penetrate the lung barrier and enter the bloodstream. Excessive ozone in the air can trigger asthma. Which of the statements given above are correct ?

Statement 1 is correct. The 2021 WHO Air Quality Guidelines recommend a 24-hour mean of PM$_{2.5}$ not exceeding 15 $\mu$g/m$^3$ and an annual mean not exceeding 5 $\mu$g/m$^3$. Statement 2 is incorrect. Ground-level ozone is formed through photochemical reactions involving sunlight, NOx, and VOCs. High concentrations typically occur during hot, sunny weather with stagnant air, not during inclement weather which tends to break down or disperse pollutants. Statement 3 is incorrect. While both PM$_{10}$ and PM$_{2.5}$ are harmful, smaller particles pose a greater health risk because they can penetrate deeper. PM$_{2.5}$ can reach the deep lung tissue and even enter the bloodstream. PM$_{10}$ mainly deposits in the upper respiratory tract and larger airways; crossing the lung barrier into the bloodstream is less characteristic of PM$_{10}$ as a whole compared to PM$_{2.5}$ and ultrafine particles. Statement 4 is correct. Excessive exposure to ground-level ozone is a known respiratory irritant and can trigger or worsen asthma symptoms, causing coughing, wheezing, and difficulty breathing.