Despite significantly reduced levels in Houston, several monitors do not yet meet the health-based standards.

Ozone is found in two regions of the Earth’s atmosphere – at ground level and in the upper regions of the atmosphere. Both types of ozone have the same chemical composition (O3). While upper atmospheric ozone protects the earth from the sun’s harmful rays, ground level ozone is the main component of smog.

Ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). Ozone is likely to reach unhealthy levels on hot sunny days in urban areas. Ozone can also be transported long distances by wind. For this reason, even rural areas can experience high ozone levels.

Ground level ozone- what we breathe- can harm our health. Even relatively low levels of ozone can cause health effects. People with lung disease, children, older adults, and people who are active outdoors may be particularly sensitive to ozone. Children are at greatest risk from exposure to ozone because their lungs are still developing and they are more likely to be active outdoors when ozone levels are high, which increases their exposure.

Ozone also affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. In particular, ozone harms sensitive vegetation, including trees and plants during the growing season.
Motor vehicle exhaust, emissions from industrial facilities and electric utilities, construction and other off-road equipment, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

o3-graph
Ozone is the only pollutant out of the six for which EPA has established national health based standard that Houston does not meet. Houston has made tremendous strides in reducing ozone over the past decade, and recently met EPA’s older standard of 84 parts per billion. Over the past two years, the extreme drought and record high temperatures caused a slight increase in ozone levels, but in 2012, levels started to decrease again. Levels of ozone’s building blocks—NOx and VOC, continued to decrease throughout this period. EPA recently adopted more stringent ozone standards—the new standard is 75 parts per billion. Emission reductions will be needed from all sources over the next few years as we work to meet this new, more stringent standard.

Source: http://www.epa.gov/groundlevelozone/basic.html