Air quality warnings are in effect for up to 100 million people in the US.
Thick smoke from Canadian wildfires to the north descended on New York this week, causing some to mask up. In some areas, people can smell the smoke in the air and the skies are hazy.
Air quality levels throughout the city are 'very unhealthy', according to the US government online platform AirNow, which also warned against outdoor activities.
An online calculator suggested breathing in the air in New York yesterday for 24 hours was equivalent to smoking six cigarettes.
Air pollution can cause breathing difficulties, as well as irritation to eyes, nose, throat and lungs. Long-term exposure to air pollution can result in chronic health issues such as severe asthma, preterm birth, heart disease, stroke, lung cancer, dementia and a lower IQ in children.
Smog can be particularly bad for young children, older adults, pregnant women and those with asthma or other respiratory conditions
The risk of respiratory infection is also increased due to the air pollution
Smoke originating from Canadian wildfires caused hazardous levels of air pollution across the Northeast. Pictured is a map showing Air Quality Index levels on Tuesday night. Purple areas are where levels were between 201 and 300. Red areas were between 151 and 200
Smog can be particularly bad for young children, older adults, pregnant women and those with asthma or other respiratory conditions. The risk of respiratory infection is also increased.
The Environmental Protection Agency uses the Air Quality Index on AirNow to report air quality.
It varies from zero to over 300, with levels 50 and below thought to be the healthiest.
When levels go above 150, the general population may start to experience symptoms.
Today's air quality in New York City is 170, which is deemed 'unhealthy'.
Inhaling this for 24 hours is the same as smoking almost five cigarettes, the online calculator claimed.
It works by using PM 2.5 particle concentration — the number of particles in the air that are a certain size.
The health impact of a particle concentration of 22μg/m3 per 24 hours is equivalent to about one cigarette.
The monitor recommends that people with heart or lung disease, older adults, children and teens should reduce their exposure by avoiding strenuous outdoor activities, keeping outdoor activities short and thinking about moving physical activities indoors or rescheduling them.
Brady Scott, a fellow at the American Association for Respiratory Care, a professional organization for respiratory therapists, told NBC he advised that people stay indoors as much as possible and keep doors and windows closed.
People should do this even while exercising, as it causes stress on the lungs.
Those with asthma should watch their symptoms closely and make sure they have medications to hand.
Mr Scott said: 'People know their bodies really well. If they see some changes they believe are related to bad air, perhaps they need to contact a physician or advanced practice provider.'
Dr Purvi Parikh, an allergist and immunologist at the Allergy & Asthma Network, an advocacy group for people with asthma, allergies and related conditions, said that smoke can be especially bad for pregnant women as their lung capacity is already reduced due to their growing stomachs.
She also said that people should keep their homes well-ventilated, and those who need to go out can wear a mask or N95 respirator.
Dr Wynne Armand, a physician at Massachusetts General Hospital and associate director of the MGH Center for the Environment and Health, said people should not dust or mow the lawn when the air quality is low.
Burning candles or using a gas stove can also make it worse.