In a typical year, there are usually two peak periods for ozone watch days: late May to early June, and then August and September.
But last Friday began a three-day run of ozone elevations, primarily due to high temperatures, lots of sunshine and little to no wind.
Bryan Lambeth is a senior meteorologist with the Texas Commission on Environmental Quality:
"In recent years have not had much activity in March, so this is fairly unusual to see high ozone in March in recent years."
On Saturday, ozone levels reached into the red zone, meaning that levels in certain locations were unhealthy for everyone, not just for people with asthma and other respiratory difficulties.
"We knew conditions were going to be close and that's why we issued the watch on Saturday. And it was surprising – we didn't expect level red we thought it would be level orange."
On Sunday, ozone levels did drop back down into the orange level.
An orange alert means levels are still a concern for vulnerable groups like people with asthma and children.
Ozone is a lung irritant produced when sunlight cooks chemicals emerging from tailpipes and factories.
Lambeth says it doesn't need to be extremely hot to create ozone; temperatures in the 70s are enough if the wind and sun conditions are right. But, that said:
"I had to go back to 1997 to find a higher eight-hour ozone average in March. So, that's the highest reading we've had in March since 1997."
Houston could see orange-level ozone readings today, but Lambeth expects the conditions to ease off during the week.