St. Louis, MO – Ozone levels are at dangerous levels in St. Louis thanks to this week's intense heat.
The federal smog standard was exceeded at three St. Louis regional air monitors this week, when temperatures edged over 100 degrees.
In all, there are 11 sites where levels are measured in the St. Louis area. Two of them in the city and one in Arnold registered unhealthy ozone.
Ozone is formed when certain gases react in the presence of sunlight. It irritates the lungs and can cause health problems.
Federal air quality rules allow no more than three exceedances at any one monitor within a three-year period.
Last year, after years of violating the federal rule, the St. Louis region narrowly complied with the rule and is now considered in attainment.