Ozone Season Guide
Ground-level ozone forms when heat and sunlight cause chemical reactions with vehicle emissions and industrial pollutants. May through September brings peak ozone levels across the US, especially during afternoon hours.
What is Ground-Level Ozone?
Ground-level ozone (O3) is different from the protective ozone layer in the upper atmosphere. At ground level, ozone is a harmful air pollutant that forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight and heat.
Primary Sources
- • Vehicle exhaust
- • Industrial emissions
- • Power plants
- • Gas-powered equipment
Formation Conditions
- • High temperatures (85°F+)
- • Strong sunlight
- • Stagnant air
- • Low wind speeds
Health Impacts
- • Lung irritation
- • Asthma attacks
- • Reduced lung function
- • Chest pain/coughing
Ozone Season Timeline
Early Season Start
Ozone Risk: Low-Moderate
Southern states begin seeing first ozone alerts. Northern regions still safe. Afternoon temperatures reach formation thresholds in Texas, California, Arizona.
Most Affected Cities
Houston, Dallas, Phoenix, Los Angeles, Bakersfield - early heat triggers formation. Coastal areas still protected by marine layer.
Peak Ozone Season
Ozone Risk: High
Maximum ozone formation nationwide. Daily alerts common in urban areas. Heat waves produce Code Orange and Code Red days. Rural areas also affected by transported pollution.
National Impact
All major metros see elevated ozone. East Coast, Midwest, and West Coast urban corridors experience frequent exceedances. Even rural areas affected by pollution transport.
Late Peak Season
Ozone Risk: High-Moderate
Still peak season but beginning to decline in northern regions. Southern states maintain high levels. Heat domes can cause spikes in any region.
Regional Patterns
Northeast and Great Lakes improving. Southeast, Texas, California still critical. Mountain West benefits from monsoon moisture.
Season Decline
Ozone Risk: Moderate-Low
Cooling temperatures reduce formation. Northern states clear by mid-month. Southern states still see alerts during heat waves. Overall improving trend.
Season End
Most cities experience significant improvement. Only desert Southwest and Southern California maintain elevated levels through month.
Cities Most Affected by Ozone
Critical Ozone Areas
- ●Los Angeles, CA - Worst in nation
- ●Bakersfield, CA - Frequent exceedances
- ●Phoenix, AZ - Extreme heat catalyst
- ●Houston, TX - Industrial + heat
High Ozone Areas
- ●Dallas-Fort Worth, TX
- ●New York-Newark metro
- ●Philadelphia, PA
- ●Sacramento, CA
Moderate Ozone Areas
- ●Atlanta, GA
- ●Chicago, IL
- ●Washington, DC
- ●Denver, CO
Daily Ozone Patterns
6 AM - 10 AM
Best time for outdoor exercise. Ozone levels at daily minimum. Cool temperatures prevent formation.
10 AM - 2 PM
Ozone forming as sun strengthens. Sensitive groups should reduce prolonged outdoor exertion.
2 PM - 6 PM
Maximum ozone levels. Avoid strenuous outdoor activity. Stay in air-conditioned spaces when possible.
6 PM - 10 PM
Ozone dissipating as sun sets. Safe for evening activities. Good alternative to afternoon exercise.
Health Protection Strategies
High-Risk Groups
- •Children: Spend more time outdoors, breathe more air per pound of body weight
- •People with asthma: Ozone can trigger attacks and worsen symptoms
- •Outdoor workers: Extended exposure during peak hours increases risk
- •Athletes: Deep breathing during exercise increases ozone intake
- •Older adults: More likely to have underlying respiratory conditions
Protection Actions
- ✓Check daily AQI forecasts before planning outdoor activities
- ✓Schedule outdoor exercise before 10 AM or after 6 PM
- ✓Move indoor workouts to gym during Code Orange or Red days
- ✓Reduce intensity of outdoor exercise on high ozone days
- ✓Keep rescue inhalers accessible during ozone season
- ✓Limit time near busy roads where pollution concentrates
Ozone Trends (2020-2024)
Progress & Challenges
Improving Areas
Northeast, Great Lakes regions showing 5-10% reduction in ozone days. Cleaner vehicles and power plants driving improvement.
Stable Areas
Mid-Atlantic, Southeast maintaining levels. Population growth offsets emission reductions. Climate factors complicating progress.
Worsening Areas
California, Southwest seeing more extreme heat events. Higher temperatures creating more ozone from same emissions.
Climate Change Impact
Rising temperatures increase ozone formation potential. Studies show each 1°F increase in temperature can increase ozone levels by 2-3%. Heat waves becoming more frequent and intense, extending ozone season and increasing peak concentrations.