Map: Watch July's wildfire smoke travel across the country

Large and unrelenting wildfires in the western United States and southern Canada are producing so much smoke that even the East Coast is feeling the effects.

These conditions can be a health risk. As smoke descended on Boston for a second time Monday, air quality conditions in parts of the city fell into the “‘unhealthy”’ category, according to data from AirNow, which publishes data indexing air quality from around the world. In New York City the same day, the air quality reached the “unhealthy for sensitive groups” category.

On July 20, New York City, Washington, D.C., and Boston awoke to a fiery sunrise and hazy skies, a result of the wildfires raging thousands of miles away. The conditions, driven by large blazes such as the Bootleg Fire in Oregon, the Tamarack and the Dixie fires in Northern California and the Snake River Complex in Idaho, have deteriorated air quality ratings in some areas.

“If you have that much smoke being put into the western atmosphere, it’s almost always going to make it to the East Coast unless something very unique happens with the wind,” said Eric James, a senior associate scientist at the National Oceanic and Atmospheric Administration’s Global Systems Laboratory.

An NBC News analysis of NOAA’s smoke data shows how the mix of large fires and wind currents carried smoke across the continental U.S.

This animation of the NOAA’s calculation of near-surface smoke shows fires in the West and in southern Canada generating large smoke plumes that accumulated over the course of July. An overlay of wind currents shows how these plumes were carried east.

Winds drew the smoke from the Western fires across the central U.S. and mixed it with the smoke generated by the fires in central Canada. Winds pulled this mixed smoke further east until, on July 20, it reached New England.

The NOAA’s smoke data is generated with an algorithm that uses fire and weather data to forecast the spread of smoke. This algorithm allows the agency to quantify how much smoke someone would experience standing on the ground.

Relief from these conditions relies on fickle weather patterns shifting winds away from the East or fires improving in Canada and the West. With no reason to expect significant rain in coming weeks, and drought and high heat driving large flames that are difficult for firefighters to contain, experts say that the worst fire may be yet to come.