[dahart]

The former; it means the average exposure over the year. The latter “always under” guideline is also included as a separate item: they recommend that your 99th percentile maximum daily exposure stay less than 15 ug/m^3 for PM 2.5, and less than 45 ug/m^3 for PM 10. 99th percentile means they recommend this level 99% of the time, and assume/allow for several days per year of exposure to higher concentrations.

There are recommendations for other specific pollutants too, I left those out, but the table includes recommendations for O3, NO2, SO2, and CO. There’s a 2nd table on the next page with recommended maximum exposures for those last 3 in terms of 1-hour and other timeframes smaller than 24 hours.