When we talk about ozone levels in Houston we are talking about 'ground level' ozone, not the ozone layer in the upper atmosphere that helps protect us from the suns rays. This ground level ozone or 'O3' for short, is mainly the result of pollutants from diesel engines (think cargo ships in the Houston ship channel and all the 18 wheelers to haul it) and coal fired power plants. These pollutants, on calm sunny days, are literally cooked by the sun and molecularly changed to form O3 or ozone.
Why does this matter to humans? It's because O3 is a lung irritant. The size of the O3 molecule is so small that it can lodge deep in the lungs and cause breathing problems. It's young children, the elderly and people with breathing problems like asthma that are most at risk. In fact I've been impacted myself, which is why I'm glad to hear that in the latest report from the American Lung Association, ozone levels in Houston have been improving.
Houston weather conditions play a big role in how much ozone is measured. On sunny days with light winds, ozone has the potential to reach unhealthy levels as the sun cooks the ozone and with no wind to blow it away it's more likely to be inhaled. On breezy cloudy days it's the opposite, with little sunshine to spark the chemical reaction that makes O3.