Ozone (O₃)

A molecule of three oxygen atoms. Protective in the stratosphere (10-50 km up — blocks UV); toxic at ground level (key component of photochemical smog). South Asian summer ozone often exceeds WHO limits.

measurement

What is Ozone?

Ozone (O₃) is a molecule made of three oxygen atoms bonded together. It is a pale-blue gas with a sharp, pungent smell — you can smell it after lightning strikes, near welding equipment, or in some air purifiers. Ozone exists naturally in the atmosphere in two distinct layers, with completely opposite effects on human health and the environment:

Stratospheric ozone (10-50 km altitude) — the “ozone layer” — protects life on Earth by absorbing harmful ultraviolet (UV-B and UV-C) radiation.
Tropospheric ozone (ground level to ~15 km) — a toxic pollutant when present in elevated concentrations, harmful to lungs, plants and materials.

This dual role is one of the most striking facts in atmospheric science: the same molecule that saves life when high up kills lung tissue when at street level.

Stratospheric ozone: the protective shield

The stratospheric ozone layer, peaking around 20-30 km altitude, contains about 90% of all atmospheric ozone. It absorbs 97-99% of medium-frequency UV (UV-B) that would otherwise reach the surface — radiation that causes skin cancer, cataracts, and disrupts ecosystems.

The ozone layer was depleted during the late 20th century by chlorofluorocarbons (CFCs) — refrigerants and propellants released by human activity. The infamous “ozone hole” over Antarctica reached peak severity in 1994 (largest October hole) and 2006 (deepest minimum).

Recovery:

The 1987 Montreal Protocol phased out CFCs globally
Stratospheric ozone is now recovering
Full recovery expected by 2066 (Antarctic), 2045 (Arctic), 2040 (mid-latitudes)
One of humanity’s clearest environmental success stories

For South Asia, healthy stratospheric ozone is critical because tropical regions receive the strongest UV radiation. The Tropic of Cancer crosses central India, putting the entire region under intense UV at noon. The UV Index regularly reaches 10-13 (Extreme) in summer — without the protective ozone layer, those values would be 20-30+.

Tropospheric ozone: the harmful pollutant

Ground-level ozone is not directly emitted by any source — it is formed in the atmosphere by photochemical reactions:

NOx + VOCs + sunlight + heat → O₃

Where:

NOx (nitrogen oxides) come from vehicles, power plants, biomass burning
VOCs (volatile organic compounds) come from gasoline vapors, paints, solvents, vegetation
Sunlight and heat drive the photochemical reaction

This is why ground-level ozone:

Peaks in the afternoon (after morning traffic + accumulated sunshine)
Peaks in summer (longer/stronger sunshine + higher temperatures)
Higher downwind of cities (pollution mixes with cleaner air)

In South Asia, summer ozone is a major air quality concern — even when PM2.5 is lower than winter, ozone fills the gap. India’s CPCB has recorded ozone levels above WHO limits in Delhi, Bengaluru, Hyderabad, Lucknow, Patna, Kanpur, Chennai.

Health effects of ozone

Ground-level ozone is a strong oxidizer that damages the airways and lung tissue:

Short-term (1-8 hour exposure):

Throat irritation, coughing
Chest pain, shortness of breath
Asthma attacks
Reduced lung function
Eye irritation
Worsened bronchitis, emphysema

Long-term exposure:

Permanent lung damage
Reduced lung development in children
Increased respiratory infections
Increased cardiovascular disease risk
Possible link to dementia (newer research)

WHO Air Quality Guideline (2021):

8-hour mean: 100 µg/m³ maximum

India CPCB standard:

8-hour mean: 100 µg/m³
1-hour mean: 180 µg/m³

Major Indian cities routinely exceed these during summer afternoons.

Ozone and other pollutants

Ozone interacts with PM2.5 and other pollutants:

Heatwave + ozone = double respiratory stress
Wildfire smoke raises NOx, fuels ozone
High humidity moderates ozone slightly (water vapor absorbs UV)
Winter inversions trap low ozone (less sunshine) but high PM2.5

This explains why summer respiratory symptoms in north India differ from winter — winter is dominated by PM2.5, summer by ozone.

Ozone monitoring

India:

CPCB CAAQMS network monitors O₃ alongside PM2.5, PM10, SO₂, NO₂, CO
200+ continuous monitors in 130+ cities
Real-time data on the National Air Quality Index (NAQI) portal

International:

WHO collects global air quality data
Satellite-derived ozone (TROPOMI on Sentinel-5P)
Open-Meteo Air Quality API includes ozone in some queries

Mausam Online displays European AQI plus PM2.5 and PM10 on every city page. Ozone is currently included in the European AQI calculation when relevant.

Protecting yourself from ozone

During high-ozone days (typically afternoons in May-October):

Limit outdoor exercise between 1 PM and 6 PM
Schedule outdoor activities for morning (before 10 AM) or evening (after 7 PM)
Check air quality before extended outdoor time
Vulnerable groups (asthmatics, children, elderly) stay indoors
Use AC when possible — recirculating air reduces ozone exposure
Avoid using ozone-generating air purifiers — counterproductive
HEPA filters do not remove ozone — only activated carbon does

Frequently asked questions

Is ozone good or bad? Both — depending on where it is. Stratospheric ozone (~20 km up) protects life by absorbing UV radiation. Ground-level ozone is a toxic pollutant. The same molecule, completely opposite roles.

Why is summer ozone worse than winter? Because ozone formation requires sunlight and heat. Summer provides both in abundance. Winter has stronger PM2.5 pollution (from heating, stagnant inversions) but weaker ozone. This is why air quality concerns shift across the year.

Is the ozone hole still there? Yes — Antarctic ozone hole still forms annually (peak October), but it’s slowly shrinking thanks to the Montreal Protocol. Recovery is expected by 2066. Arctic and mid-latitude ozone are recovering faster.

Does ozone affect Indian agriculture? Yes. Studies estimate India loses 5-15% of staple crop yields (wheat, rice) to ozone exposure. Karnataka, Telangana and Maharashtra wheat regions are particularly affected.

Where can I see live air quality for my city? Mausam Online displays European AQI and PM2.5/PM10 on every city page. See Delhi, Mumbai, Chennai, Bengaluru, Hyderabad, Lucknow.