What is Ground-Level Ozone?
Ground-level ozone (O3) is a colorless gas and the main ingredient in smog. Unlike stratospheric ozone (the good ozone layer that protects us from UV rays), ground-level ozone is harmful to human health and the environment.
Ozone is not emitted directly into the air. It forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. This is why ozone levels are typically highest on hot, sunny days.
Sources of Ozone Precursors
Ground-level ozone forms from pollutants emitted by:
- Cars, trucks, and buses (major source of NOx)
- Power plants and industrial facilities
- Gasoline vapors and chemical solvents
- Paints, cleaners, and other consumer products
- Natural sources like trees and vegetation (VOCs)
Health Effects of Ozone
Breathing ozone can trigger a variety of health problems, especially in sensitive groups:
Respiratory Effects
- Coughing and throat irritation
- Chest pain when breathing deeply
- Shortness of breath
- Reduced lung function
- Aggravated asthma symptoms
- Increased susceptibility to respiratory infections
- Inflammation of the lungs
Who is Most at Risk?
- Children, whose lungs are still developing
- Adults who are active outdoors
- People with asthma or other respiratory diseases
- Older adults with declining lung function
- People with certain genetic characteristics
- Outdoor workers
When Ozone Levels are Highest
Ozone pollution follows predictable patterns:
- Time of day: Peaks in the afternoon when the sun is strongest (typically 2-6 PM)
- Season: Summer months have the highest levels (ozone season)
- Weather: Hot, sunny, stagnant conditions increase ozone formation
- Location: Urban and suburban areas downwind of cities
AQI Levels for Ozone
| AQI Range | Category | Health Concern |
|---|---|---|
| 0-50 | Good | Little or no risk |
| 51-100 | Moderate | Sensitive groups may experience symptoms |
| 101-150 | Unhealthy for Sensitive Groups | Sensitive groups should limit prolonged outdoor exertion |
| 151-200 | Unhealthy | Everyone may experience health effects |
| 201-300 | Very Unhealthy | Health alert: everyone may experience serious effects |
| 301+ | Hazardous | Emergency conditions: entire population affected |
Protecting Yourself from Ozone
Check the Forecast
- Monitor daily AQI forecasts for ozone levels
- Plan outdoor activities for morning hours when ozone is lower
- Avoid strenuous outdoor activities on high ozone days
During High Ozone Days
- Limit time outdoors, especially during afternoon hours
- Avoid exercising near high-traffic areas
- Keep rescue medications handy if you have asthma
- Stay in air-conditioned spaces when possible
- Watch for symptoms like coughing, chest discomfort, or difficulty breathing
Reduce Your Contribution
- Drive less, carpool, use public transit, or bike
- Refuel your car in the evening when its cooler
- Use low-VOC paints and cleaning products
- Dont idle your car
- Keep your car well-maintained
- Use electric lawn equipment instead of gas-powered
Ozone vs. Particulate Matter
Ozone and particulate matter (PM2.5) are the two main components of air pollution, but they differ significantly:
- Ozone is a gas formed by chemical reactions in sunlight; it irritates the respiratory system and is highest in summer afternoons
- PM2.5 consists of tiny solid particles that can penetrate deep into lungs; its sources include combustion, dust, and industrial processes
- Both can cause health problems, but they require different protection strategies
- N95 masks filter particles but do not protect against ozone