In the context of WHO Air Quality Guidelines, consider the following statements:

1. 1.The 24-hour mean of PM2.5 should not exceed 15 micrograms per cubic metre and its annual mean should not exceed 5 micrograms per cubic metre.
2. 2.The highest ozone-pollution levels occur during inclement weather.
3. 3.PM10 can penetrate the lung barrier and enter the bloodstream.
4. 4.Excessive ozone in the air can trigger asthma.

Which statements are correct?