'Stay indoors,' urges NY governor as smoke shrouds NYC

STORY: Hochul also called the situation an "emergency crisis," noting that the air pollution index for parts of her state were eight times above normal.

In some areas, the Air Quality Index (AQI), which measures major pollutants including particulate matter produced by fires, was well above 400, according to Airnow, which sets 100 as "unhealthy" and 300 as "hazardous."

''Brooklyn in the last hour was at 413. This is, again, safe level is 50. This is 415 health warnings for everyone. The entire population is likely to be affected. Queens right behind Brooklyn. So we're seeing these effects. That's 407 in the Queens area,'' Hochul said. ''So our message right now is going to be reiterated multiple times because it is simply stay indoors,'' she added.

The smoke was wafting over the U.S. northern border from Canada, where hundreds of forest fires have scorched 9.4 million acres (3.8 million hectares) and forced 120,000 people from their homes in an unusually early and intense start to Canada's wildfire season.

Wildfire smoke has been linked with higher rates of heart attacks and strokes, increases in emergency room visits for asthma and other respiratory conditions, and eye irritation, itchy skin and rashes, among other problems.

Poor air quality is likely to persist into the weekend, with a developing storm system expected to shift the smoke westward across the Great Lakes and deeper south through the Ohio Valley and into the mid-Atlantic region, AccuWeather said.