Air quality is typically the worst around 6 p.m. in Sacramento area during the summer. This is due to heating of gases creating higher ozone.
Fireworks on the 4th of July can present health hazards later in the day. Many times, the worst air quality on the holiday is later in the evening around 11 p.m.
The issue isn't ozone, it is particulate matter (PM) from fireworks. In large quantities, this can be harmful, especially to those with existing respiratory problems.
Air quality usually spikes around 11 p.m. in the very unhealthy range on the air quality index range. The good news is it falls within about an hour, but is still in the unhealthy range.
Air quality tends to be the best in the early morning hours.