PICHER, Okla. - U.S. officials plan to test the air in what's left of a heavily polluted former mining town in Oklahoma after it was hit by a powerful tornado.

The tornado was one of several that combined to kill 22 people in the U.S. Midwest and the South over the weekend.

The storms raised this year's death toll to about 100, the worst in a decade and on pace to become the worst since 130 people were killed in 1998. The record is 519 tornado-related deaths in 1953.

In Picher, the devastation has been complicated by dust blown off giant mounds of lead-filled waste left over from non-defunct mining operations.

Miles Tolbert, Oklahoma's secretary of the environment, says he doesn't think the town's 800 residents face an immediate health hazard. But he says more testing is needed to be certain.

Long-term exposure to lead dust poses a health risk, particularly to young children.

On Saturday, a powerful tornado killed six people in Picher as it destroyed a 20-block area and blew dust off mountains of mining waste, or chat piles.

"You can look at the chat piles and see that a lot of the material has blown off," said John Sparkman, head of the Picher housing authority.

"We went up on a chat pile an hour and a half after the tornado hit, and you could see dust blowing fine material all over the place from that vantage point."

In all, 22 people were killed by a rash of tornadoes in Oklahoma, Missouri and Georgia.

Meanwhile, law enforcement officers and the Oklahoma National Guard patrolled the Picher overnight into Monday to prevent looting, said Michelann Ooten, spokeswoman for the Oklahoma Department of Emergency Management.

National Weather Service assessment teams determined the twister that hit Picher had an EF-4 rating, the second highest rating, and was about 1.6-kilometres wide at its widest point, meteorologist Mike Teague said Monday.

The tornado's winds were estimated at 265 to 280 kilometres an hour. The damage track stretched 119 kilometres through Oklahoma and Missouri, where 15 people were killed.

"These storms are fairly rare to be that strong. The devastation was nearly complete in a few areas," Teague said.

"Albeit isolated, there were some sections of neighbourhoods where houses were just completely taken off the foundation. Gone."

The tornado could be the ultimate incentive for those 800 or so residents who have been reluctant to leave, now that most of their homes have been ruined, Sparkman said.

One of those residents, Sue Sigle, had been hoping the government would offer more money for her home before she moves away from this pollution-scarred town. Then the tornado came.

As she began the task of salvage Sunday, Sigle kept a smile on her face, noting that she was fortunate to be visiting family in Missouri when the massive twister hit.

"I'm OK with everything," Sigle said. "The Lord is going to take care of anything. ... I was going to move anyway. I guess I'll just have to move sooner."

That sense of inevitability appeared to grip residents as they picked through the remnants of their homes. The lead and zinc mines that made Picher a booming town of about 20,000 in the mid-20th century closed decades ago; leftover waste has turned the area into an environmental disaster.

Gov. Brad Henry, who toured the area by air and on foot Sunday, said the buyout program won't stop just because homes were levelled, adding that he would "guarantee" that those awaiting buyouts who lost their homes would be treated fairly.

"We will make sure the people get the assistance that they need," Henry said.

Because of Picher's pollution status, U.S. Federal Emergency Management Agency is unlikely to grant assistance to homeowners to rebuild in the town, said Oklahoma Emergency Management Director Albert Ashwood.

But he echoed Henry's assurances about the federal buyout program, which is funded by the Environmental Protection Agency.

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