More than 170 million Americans are under either heat or air quality alerts as wildfire smoke and scorching temperatures continue to plague the US.

Air quality across a large part of the Midwest and Mid-Atlantic has been deemed "unhealthy" by health officials.

Meanwhile, in the west, cities including Las Vegas and Phoenix are bracing for temperatures of up above 43C (110F) this weekend.

At least a dozen people have died of heat-related causes in recent days.

The poor air quality has been attributed to over 500 active wildfires in Canada that have sent smoke plumes south, even as conditions have improved slightly in parts of the Midwest.

On Friday, the country's worst air quality was expected across a vast tract of land stretching from Michigan and Ohio to the East Coast.

The worst air quality in the US at approximately 09:00 local time (14:00 BST) on Friday was in New York City, followed by Washington DC, Detroit and Chicago, according to IQAir.com.

The Environmental Protection Agency said air in these area was either "unhealthy for sensitive groups" - which include the elderly, young children and outdoor workers - or "unhealthy".

Authorities have declared a state-wide air quality advisory that will remain in effect through Friday. Governor Kathy Hochul urged that New Yorkers "take precautions to protect their health".

In the Washington DC area, concerns over air quality prompted the cancellation of dozens of outdoor events and a warning from local health authorities to avoid strenuous outdoor activities and to consider wearing a mask when outside.

In Canada, parts of the provinces of Ontario, Quebec are most affected by the smoke.

In the Toronto, Ottawa and Montreal regions, poor air quality is expected to persist into the weekend, with conditions beginning to improve into Saturday.

Meanwhile, in the western US, millions of people in Nevada, Arizona and California can expect excessive heat warnings beginning on Saturday, with temperatures in some places expected to rise to 43C.

In Texas, which has seen temperatures above 37C for nearly two weeks, temperatures have begun to drop. Eleven people have reportedly died of heat-related causes in the state's Webb County, which is centred on the city of Laredo along the Mexican border.

"Although a few locations across Central and East Texas may reach advisory criteria, the prevalence of 105F [43C] heat indices will be far less widespread, and our protracted stretch of advisories/warnings for heat will come to an end," the National Weather Service said in a bulletin.

Storms that are expected in other parts of the south are also expected to bring temperatures down over Saturday and Sunday.

Over 700 people die heat-related deaths each year, according to CDC statistics, with another 68,000 people sent to emergency rooms.

Heatwaves have become more frequent, intense and last longer because of human-induced climate change, scientists say.

Some have warned that climate change is also likely to lead to more wildfires and subsequent smoke warnings.

A University of California - Irvine study published on 12 June, for example, found that "an increase in temperatures and dryness has been identified to be one of the major drivers" of summer forest fires.