More than 830 wildfires are currently burning in Canada. (AP Photo used for representation)A massive plume of smoke from wildfires burning across Canada has swept into the United States, degrading air quality across the Midwest, Northeast and Mid-Atlantic.The smoke, carried south by prevailing winds, is expected to affect millions of people through the end of the week, with health officials warning residents to limit outdoor activities as pollution levels climb.Smoke spreads across multiple statesMore than 830 wildfires are currently burning in Canada, with some of the largest blazes raging through dense forests in west-central Ontario, ABC News reported.The fires are producing enormous volumes of smoke that have crossed the border into the US.Northern Minnesota, where more than a dozen wildfires are also burning with little or no containment, is adding to the smoke burden.The thickest smoke initially covered parts of Minnesota, Wisconsin and Michigan before spreading eastward. By Wednesday, smoke had reached New York state and New England, with forecasts indicating hazy conditions from Buffalo to New York City, Philadelphia and Boston.Cities including Detroit, Milwaukee, Cleveland, Columbus, Baltimore and Washington, DC, are also expected to experience degraded air quality.Health risks and advisoriesAuthorities have issued air quality alerts across several states, including all of Michigan and Wisconsin, warning that fine particulate matter from wildfire smoke can cause breathing problems, aggravate heart and lung diseases and pose risks even to otherwise healthy individuals when pollution reaches hazardous levels.Health experts advise residents to stay indoors as much as possible, keep windows closed, use air purifiers or air conditioning with high-efficiency filters and wear N95 masks if outdoor exposure is unavoidable.Why the smoke is travelling so farAccording to meteorologists, upper-level winds and stagnant weather conditions have allowed smoke to spread across a vast region while a heat dome has prevented it from dispersing quickly.Rain forecast later this week over parts of the Midwest and Northeast could provide some relief by clearing the smoke.Scientists say hotter temperatures, prolonged drought and increasingly dry forests are making wildfires more frequent and intense across North America.