What creates smog in New York City

NYC smog is not a single pollutant but a visible haze caused by nitrogen oxides and volatile organic compounds reacting with sunlight. The reaction creates ground-level ozone, the main ingredient in what you see as a brownish or grayish cloud hanging over the city. On any given day, the smog you see depends on three things: how much pollution cars, trucks, and buildings are releasing; how hot it is (heat speeds up the chemical reaction); and whether wind is pushing the pollution away or trapping it over the city.

The largest source of nitrogen oxides in NYC is vehicle exhaust — cars, buses, and trucks moving through the five boroughs. Power plants, refineries, and industrial facilities contribute as well. Volatile organic compounds come from vehicle fuel, paint, cleaning products, and gas stations. When these chemicals sit in strong sunlight on a warm day, they transform into ozone, which is what makes the air hazardous to breathe.

Weather patterns determine whether smog builds up or disperses. On days when high pressure sits over the Northeast and wind is light, pollution gets trapped in the city and smog thickens. In summer, when temperatures are highest, the chemical reaction happens fastest. This is why smog alerts are most common from June through September, though they can occur in other months.

Key Takeaways

  • NYC smog forms when vehicle exhaust and industrial emissions react with sunlight to create ground-level ozone, visible as a brownish haze.
  • The Air Quality Index (AQI) runs from 0 to 500, with readings above 100 considered unhealthy and above 150 considered very unhealthy.
  • You can check real-time air quality for your neighborhood on the New York State Department of Environmental Conservation website or the EPA's AirNow tool.
  • People with asthma, heart disease, or respiratory conditions should limit outdoor activity when the AQI exceeds 100, and everyone should reduce exertion when it exceeds 150.
  • Smog is worst on hot, windless days in summer, and forecasts typically appear 24 hours in advance on the DEC website.

Reading the Air Quality Index for your neighborhood

The Air Quality Index (AQI) is a number from 0 to 500 that tells you how polluted the air is right now. The EPA calculates it based on five pollutants: ground-level ozone, particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide. In NYC, ozone is usually the main driver of the AQI on smog days. The higher the number, the worse the air quality and the greater the health risk.

The AQI is divided into color-coded ranges. Green (0–50) means air quality is good. Yellow (51–100) means it is moderate — sensitive groups should consider limiting prolonged outdoor exertion. Orange (101–150) means air quality is unhealthy for sensitive groups, and anyone with asthma or heart disease should reduce time outdoors. Red (151–200) means unhealthy air for the general population, and most people should limit outdoor activity. Purple (201–300) and maroon (301+) mean very unhealthy to hazardous air, and outdoor activity should be avoided.

The AQI varies by neighborhood because pollution concentrations differ across the city. Areas near highways, truck routes, and industrial zones often have higher readings than residential neighborhoods farther from major emission sources. Check the reading for your specific area rather than assuming the whole city has the same air quality.

Where to find real-time air quality data for NYC

The New York State Department of Environmental Conservation (DEC) publishes real-time air quality readings for monitoring stations across the city. You can visit their website and select your borough or neighborhood to see the current AQI and which pollutants are elevated. The data updates hourly.

The EPA's AirNow website (airnow.gov) also displays current air quality for NYC and allows you to enter your zip code to see readings specific to your area. The tool shows the AQI, the primary pollutant driving that number, and a forecast for the next two days. The EPA also runs an AirNow mobile app for iOS and Android.

The New York City Department of Environmental Protection (DEP) sends air quality alerts through its website and social media when the AQI is forecast to exceed 100. You can also sign up for text or email alerts through the DEC website. These notifications typically arrive 24 hours before a smog event, giving you time to plan outdoor activities or take precautions.

Health effects of smog at different AQI levels

At an AQI of 51–100 (yellow/moderate), most people can breathe normally, but people with asthma, emphysema, or heart disease may experience symptoms like coughing or shortness of breath during outdoor exertion. Children and older adults are also more sensitive. If you fall into a sensitive group, reduce strenuous outdoor activity on yellow days.

At an AQI of 101–150 (orange/unhealthy for sensitive groups), sensitive individuals should avoid prolonged outdoor exertion. Symptoms can include chest pain, wheezing, and difficulty breathing. The general population usually does not experience symptoms at this level, but some people may notice irritation in their eyes or throat.

At an AQI of 151–200 (red/unhealthy), the general population begins to experience health effects. Healthy adults may develop coughing, shortness of breath, or chest discomfort during outdoor activity. People in sensitive groups should stay indoors or limit activity to light, short-duration tasks. At an AQI above 200 (purple/very unhealthy or maroon/hazardous), everyone should avoid outdoor exertion, and vulnerable populations should remain indoors with windows closed.

Long-term exposure to smog increases the risk of asthma, bronchitis, and reduced lung function, particularly in children whose lungs are still developing. People with existing respiratory or cardiovascular disease face higher risks of hospitalization or death on high-pollution days.

Steps to reduce your exposure on high-smog days

When the AQI exceeds 100, limit outdoor time, especially strenuous activity like running, cycling, or sports. If you must go outside, avoid peak pollution hours, which are usually late afternoon and early evening when the sun is strongest and ground-level ozone peaks. Early morning air is often clearer.

Keep windows and doors closed on high-smog days to prevent outdoor air from entering your home. If you have air conditioning, run it on recirculate mode so it filters indoor air rather than pulling in outdoor air. HEPA filters in air purifiers can remove some particulate matter, though they do not remove gases like ozone.

If you have asthma or another respiratory condition, keep your rescue inhaler with you and follow your doctor's guidance on activity restrictions. People with heart disease should also consult their doctor about whether to adjust exercise routines on high-pollution days. Children should be supervised to may support they do not overexert themselves outdoors.

Wear an N95 or P100 mask if you must spend time outdoors on a red or purple AQI day. Surgical masks and cloth masks do not filter ozone or fine particles effectively. Make sure the mask fits snugly around your nose and mouth to be effective.

Long-term trends in NYC air quality

NYC air quality has improved significantly since the 1970s, when smog was far more severe and frequent. The Clean Air Act, vehicle emission standards, and the phase-out of coal-burning power plants have all contributed to cleaner air. However, ozone pollution remains a problem on hot summer days, and the city still exceeds federal air quality standards on some days each year.

Climate change is making smog management harder. Warmer temperatures speed up the chemical reaction that creates ozone, and heat waves are becoming more frequent and intense. This means smog events may become more common and more severe in coming decades, even if emissions remain constant.

The city continues to work on reducing emissions through vehicle electrification, building efficiency standards, and industrial regulations. The Congestion Pricing program, which began in 2024, is intended to reduce vehicle traffic in Manhattan and lower emissions. However, these changes take years to show measurable effects on air quality.

Frequently Asked Questions

Is smog the same as air pollution?

Smog is a visible type of air pollution caused by ground-level ozone and particulate matter reacting with sunlight. Air pollution is a broader term that includes all harmful substances in the air, including gases you cannot see. You can have poor air quality without visible smog, and you can see smog when ozone levels are high.

Can I exercise outdoors when the AQI is yellow?

If you are healthy, light to moderate exercise is usually safe at a yellow AQI (51–100). If you have asthma, heart disease, or are over 65, reduce strenuous activity or exercise indoors instead. Listen to your body — if you feel chest pain, wheezing, or unusual shortness of breath, stop and move indoors.

Do air purifiers remove ozone?

Most HEPA filters remove particulate matter but not ozone gas. Some purifiers use activated carbon filters, which can absorb ozone, but they need to be replaced regularly. The most effective way to reduce ozone exposure is to stay indoors with windows closed and air conditioning on recirculate mode.

How far in advance can smog be forecast?

The EPA and DEC typically forecast air quality 24 to 48 hours in advance. Forecasts are most accurate one day out. On days with uncertain weather patterns, forecasts may be less reliable. Check the forecast the day before to plan outdoor activities.

Does rain clear smog from the air?

Rain removes particulate matter and some pollutants from the air, so air quality often improves after rainfall. However, rain does not remove ozone gas — ozone is destroyed by chemical reactions in the atmosphere or by reacting with surfaces, not by precipitation. Air quality may still be poor even after rain if ozone levels are high.