Where New York City's smog comes from

New York City's smog is not a single source — it comes from vehicle exhaust, power plants, industrial facilities, and construction sites within the city, plus pollution that drifts in from other states. The nitrogen oxides and volatile organic compounds released by cars and trucks react with sunlight to form ground-level ozone, which is the main ingredient in smog. On hot, still days when air does not move, these pollutants build up instead of dispersing, and smog becomes visible and thick.

A significant portion of New York City's smog originates upwind — from power plants and refineries in New Jersey, Pennsylvania, and Ohio. When weather patterns push that pollution eastward, it arrives in the city already formed or ready to form. This is why smog in New York City often worsens in summer, when heat and sunlight are strongest and when high-pressure weather systems trap air over the region.

The city's geography makes the problem worse. The Hudson River valley and the surrounding terrain can trap air, preventing it from moving out to sea. On days when the wind is light or coming from the wrong direction, pollutants accumulate rather than clear.

Key Takeaways

  • New York City's smog forms when vehicle exhaust and industrial emissions react with sunlight, and it worsens on hot, still days when air cannot disperse.
  • Pollution from power plants and refineries in neighboring states drifts into the city and contributes significantly to smog, especially in summer.
  • The Air Quality Index (AQI) tells you how polluted the air is on any given day, and you can check it before outdoor activities.
  • People with asthma, heart disease, or lung conditions face the highest health risks from smog, but anyone can experience symptoms on bad air days.
  • Reducing vehicle trips, staying indoors on high-pollution days, and using air filters can lower your personal exposure to smog.

How the Air Quality Index measures smog in New York City

The Air Quality Index, or AQI, is a number from 0 to 500 that tells you how clean or polluted the air is. The Environmental Protection Agency (EPA) calculates it based on five major pollutants: ground-level ozone, particle pollution, nitrogen dioxide, sulfur dioxide, and carbon monoxide. In New York City, ozone and particle pollution are usually the main drivers of the AQI on smoggy days.

An AQI below 50 is considered good air quality. Between 51 and 100 is moderate — sensitive people may notice symptoms, but the general public is usually unaffected. Above 100, the air is unhealthy for sensitive groups. Above 150, it becomes unhealthy for everyone. Above 200, it is very unhealthy, and above 300, it is hazardous.

You can check the current AQI for New York City on the EPA's AirNow website (airnow.gov) or through the New York State Department of Environmental Conservation website. Both update the forecast several times a day. Many weather apps also display the AQI alongside temperature and humidity.

Health effects of smog exposure

Smog irritates the airways and lungs. When you breathe it in, it can trigger coughing, wheezing, shortness of breath, and chest pain. These symptoms are most noticeable during or shortly after outdoor activity, because exercise increases the amount of air you breathe in. People with asthma, emphysema, chronic bronchitis, or heart disease are at highest risk — smog can send them to the hospital on bad air days.

Children and older adults are also more vulnerable, even without a diagnosed lung or heart condition. Children's lungs are still developing, and older adults often have reduced lung function. Pregnant people face risks too, because reduced oxygen intake can affect fetal development.

Healthy adults can experience symptoms on very polluted days, especially if they exercise outdoors. Symptoms usually fade once you move to cleaner air, but repeated exposure over time may cause lasting damage to lung function. People who work outdoors — construction workers, delivery drivers, park maintenance staff — face higher cumulative exposure.

When smog is worst in New York City

Smog peaks in summer, typically from June through September. Heat and strong sunlight speed up the chemical reactions that form ozone. High-pressure weather systems that settle over the Northeast trap air and prevent it from moving, allowing pollutants to accumulate. Days with temperatures above 85°F and light winds are the highest-risk days.

Smog can also spike in early fall, particularly in August and September, when summer heat lingers but weather patterns begin to shift. Winter smog is less common in New York City because cold temperatures and cloud cover slow ozone formation, though particle pollution from heating and vehicle exhaust can still be a problem on still, cold days.

Time of day matters too. Ozone concentrations are usually highest in the afternoon and early evening, after hours of sunlight have had time to drive the chemical reactions. Morning and night typically have lower ozone levels, though particle pollution can remain elevated around the clock.

Steps you can take to reduce your exposure

On days when the AQI is above 100, limit outdoor activity, especially strenuous exercise. If you must go outside, do it in the morning or evening when ozone levels are lower. Keep windows closed on high-pollution days and use an air conditioner with a clean filter, or a portable HEPA filter indoors.

If you have asthma or another respiratory condition, keep your rescue inhaler with you and follow your doctor's guidance on activity limits during high-pollution days. Some people benefit from wearing an N95 mask outdoors on smoggy days, though masks do not filter out all pollutants and are most effective for particle pollution rather than ozone.

Reducing vehicle trips — by using public transit, biking, or working from home when possible — lowers the emissions that form smog. Carpooling, combining errands into one trip, and avoiding idling all reduce your contribution to the problem. These actions benefit not just your own air quality but the air quality for everyone in the city.

How New York City and the EPA work to reduce smog

New York State has adopted vehicle emission standards stricter than the federal minimum, which means cars sold in New York must produce fewer pollutants. The state also regulates power plants and industrial facilities, requiring them to install pollution-control equipment and meet emission caps.

The EPA sets the National Ambient Air Quality Standards (NAAQS) for ozone and other pollutants. When an area fails to meet these standards — which New York City has in recent years — the EPA designates it as a "nonattainment area" and requires the state to develop a plan to improve air quality. New York's plan includes stricter rules on vehicle emissions, industrial operations, and construction dust.

The city also runs its own air quality monitoring network with sensors at multiple locations throughout the five boroughs. This data feeds into the AirNow forecast and helps the city and state track whether air quality is improving or worsening over time.

Frequently Asked Questions

Is the smog in New York City getting worse?

Air quality in New York City has improved significantly since the 1970s, when smog was far more severe. However, ozone levels have plateaued or worsened in recent years, partly because of climate change — higher temperatures favor ozone formation — and partly because pollution from upwind states continues to drift in. The city meets federal ozone standards in some years and exceeds them in others.

Can I exercise outdoors on a day when the AQI is 120?

An AQI of 120 is unhealthy for sensitive groups but not necessarily for healthy adults. However, if you have asthma, heart disease, or lung problems, you should avoid strenuous outdoor exercise. Healthy people can usually exercise outdoors at this level, but you may notice symptoms like throat irritation or coughing. If you do exercise, do it in the morning or evening when ozone is lower.

Does an air purifier in my home actually help?

A HEPA filter removes particle pollution (dust, pollen, smoke) but does not remove ozone or gases. If your main concern is ozone on smoggy days, keeping windows closed and using air conditioning is more effective than a purifier. A purifier is most useful if you live near a highway or construction site, or if you have pets or allergies.

Why does New York City have smog when I do not see it?

Ozone smog is not always visible — you cannot see the gas itself. Visible smog is usually particle pollution (haze), which forms from dust, soot, and chemical reactions. You can have unhealthy ozone levels on a clear day, which is why checking the AQI is more reliable than looking out the window.

What should I do if I have an asthma attack during high smog?

Use your rescue inhaler as directed by your doctor. Move indoors to cleaner air if possible. If your symptoms do not improve within 15 minutes, or if you have difficulty breathing, chest pain, or confusion, call 911. Keep your doctor informed of smog-related attacks so they can adjust your treatment plan.