Much of the United States felt like a blazing inferno on Wednesday, as record heat attacked the South like a blowtorch, thick smoke from Canadian wildfires blanketed the Great Lakes region, and triple-digit temperatures threatened to wallop California for the first time this year.

Scientists said climate change helped shape the weather conditions that were causing misery and putting lives at risk from Mexico to Canada. There was no disputing the impact: If it wasn’t way too smoky, it was way too hot.

“Everybody’s saying, ‘We’re used to the heat, but not to this degree,” said Mayor Victor Treviño of Laredo, Tex., which recorded temperatures of 115 degrees last week, tying its all-time high. There were nine heat-related deaths, Webb County Medical Examiner Corinne Stern told county commissioners Monday, adding that “I think our county was caught a little off-guard.”

If Laredo and elsewhere in the South were caught off-guard by the heat, much of the rest of the country was getting an unexpected dose of horrendous air quality because of dense wildfire smoke rolling in from Canada. Air quality alerts related to the smoke were in effect for parts of some 17 states, covering nearly a third of the U.S. population.

Detroit, Chicago and Minneapolis were among the cities with the worst air quality in the world Wednesday, according to IQAir. Unhealthy Code Red and Purple conditions stretched from eastern Iowa across Chicago and the lower Great Lakes region and toward the Appalachian Mountains, according to AirNow.gov.

“Poor air quality due to smoke from Canadian wildfires will continue through Thursday,” the National Weather Service in Chicago wrote on Twitter. “Everyone should avoid prolonged outdoor activities and those w/chronic respiratory issues should stay indoors if possible.”

An exceptionally rare Code Maroon was recorded in Decatur, Ill., during the midday. This is the worst level on the scale and considered hazardous. Chippewa, Ohio, was also reporting Code Maroon conditions.

Cities seeing Code Purple air quality Wednesday — very unhealthy, with an increased health risk for the general public — included Chicago, Detroit, Cleveland, Pittsburgh, Milwaukee, Indianapolis and Cedar Rapids in Iowa.

Forecasts for the smoke were changing, sometimes rapidly. After lingering in Code Orange territory on Wednesday, D.C., Maryland and Virginia were looking at more-dangerous Code Red conditions for Thursday, the Metropolitan Washington Council of Governments said on Twitter on Wednesday afternoon.

Between the wildfire smoke and the heat, “You get to pick your poison,” said Andrew Dessler, a professor of atmospheric sciences at Texas A&M University.

“They’re different things, but the common factor is climate change,” Dessler said. “Heat waves occur naturally, as do fires, but climate change makes the heat waves more intense and the fires more intense.”

Such phenomena are likely only to worsen in years to come, but a gridlocked Congress has no plans to take action on climate beyond the Inflation Reduction Act passed last year. That leaves further efforts up to states, where approaches vary wildly. Some liberal states, particularly California, have moved aggressively to pass climate legislation, while conservative states such as Texas have taken little or no action.

“We are seeing a climate that didn’t exist before,” said Alice Hill, senior fellow at the Council on Foreign Relations, on Wednesday at a Washington Post Live event on extreme weather. “We simply don’t have what we need for a climate-worsened environment, and we are sorely behind in the land use choices, the building codes, the type of changes that would keep us much safer in a hotter, more dangerous world.”

Relief from the wildfire smoke and the Southern heat wave was on the horizon, but not for another several days.

The National Weather Service in Houston warned that heat indexes above 108 threatened into the weekend.

“If you’ve been thinking to yourself, ‘Self, it’s felt like a raging inferno for awhile . . . ‘ Well, here’s some reassurance that it has indeed been feeling spicy for what seems like an eternity. Heat Indices have been in the triple digits for most of June,” the agency said Tuesday night on Twitter.

According to tracking by The Washington Post, 59 million people in the United States were at risk of being exposed to dangerous heat on Wednesday, mostly in Texas and other Southern states, including Florida.

Officials in multiple cities pleaded with members of the public to stay indoors, conserve energy if possible and make use of cooling centers that were opening in public libraries in Dallas, Houston and elsewhere.

“Anyone is at risk for a heat emergency this year because we have such high heat so early in the year,” David Persse, chief medical officer for the Houston Health Department, said Wednesday in an interview on NBC affiliate station KPRC2 in Houston.

Preliminary figures from the Electric Reliability Council of Texas, which operates the grid serving most of the state, indicated that demand between 5 and 6 p.m. Tuesday exceeded the previous record set in July 2022.

San Antonio Fire Department spokesman Woody Woodward said there had been a 53 percent increase in heat-related calls so far this June, compared with the same time period a year ago.

“I don’t ever recall seeing it so many consecutive days that it feels like 110, 112 and 114,” Woodward said in an interview.

Numerous temperature records have been broken in Texas since the heat wave began more than three weeks ago.

Temperatures have routinely topped 100 degrees across much of the state, including in Rio Grande Village in southwest Texas near the Mexico border, which reached an all-time high of 118 degrees. In Corpus Christi, the heat index reached a blistering 125 degrees.

The National Weather Service said the heat would expand from Texas into the lower Mississippi Valley and mid-South through Thursday.

Excessive heat warnings and heat advisories were in effect Wednesday across parts of 14 states. The hottest zone Wednesday stretched from Central Texas northward through central Oklahoma into southeast Kansas and southwest Missouri, where many locations were expecting high temperatures at least 10 to 15 degrees above normal.

Temperatures were forecast to reach near or past 100 degrees across much of Texas, southern New Mexico, Oklahoma, Kansas, Arkansas and Louisiana on Wednesday through Friday, with heat indexes climbing as high as 110 to 120 degrees.

Dangerous heat was expected to expand deeper into Missouri, Mississippi, Illinois, Tennessee, Kentucky, Alabama and the Florida Panhandle on Thursday, and into Georgia this weekend.

“Many areas outside of Texas will experience their most significant heat of the season,” the National Weather Service said in a summary of key messages about the heat wave. “Increased heat-related danger persists this week due to the longevity of this ongoing heat wave with record high nighttime lows and elevated heat index readings during the day. It is essential to have ways to cool down and limit your heat exposure.”

Forecasters said the heat dome — a large high pressure zone that traps heat underneath it — responsible for the heat wave is expected to weaken late this weekend into next week, which should allow temperatures to trend a bit cooler during the week of July Fourth.

But if the South is in line for a letup, other places may get hit instead. After months of relatively cool temperatures, California was set to face its first major heat wave of the season, with triple-digit temperatures threatening the state’s Central Valley.

The exceptionally wet winter California enjoyed has seeped into the soil and plant life, counteracting fire risk somewhat, officials said. At the same time, the record snowpack has produced swift currents in area rivers, leading the Weather Service to caution residents to be careful about trying to cool off in local bodies of water, many of which also remain frigid after the cold winter.

The Washington Post’s Ian Livingston and Matthew Cappucci contributed to this report.