On the morning of May 19, 1953, a dry lake bed at Yucca Flat, Nev. cracked under a devastating explosion. A bright orange fireball climbed into the sky, dissolved into a purplish mushroom cloud, then floated eastward on the wind. Moments after the blast, the residents of St. George, Utah—145 miles away—felt the ground beneath them tremble. A few hours later, a gray ash fell from the sky, coating their pastures, clinging to laundry and burning the skin of people it touched.

Known locally as "Dirty Harry," the atom bomb that caused the fallout was not the first to leave its mark on St. George, though at 32 kilotons, it was one of the largest. From 1951 until the 1963 nuclear test-ban treaty, the Atomic Energy Commission set off at least 100 aboveground devices at the Nevada testing site. Yet, though herds of sheep and pigs in St. George fell dead within days of Dirty Harry, the AEC ignored those who claimed any connection between fallout and injury to man or beast. For decades, the government has clung to this position, and, for almost as long, one St. George woman, Irma Thomas, 72, has waged a quiet but tenacious battle to prove the bureaucrats wrong. Says Thomas: "All I ever wanted to do was let the government know what they did to the people of St. George."

Her struggle may be nearing an end at last. Reputable scientists now suspect that the tests caused a phenomenally high rate of cancer and thyroid diseases among residents of St. George. They have also linked them to a variety of other problems; one researcher has even theorized that the fallout may have caused a decline in SAT scores among Utah high school students. The federal government no longer flatly denies such dire possibilities. Spurred in part by Irma Thomas' efforts, 442 victims and their families have sued the government, charging negligence and failure to warn the residents of the danger they faced and demanding a reported $230 million. "We were used as fodder, the same as our young men were used in Vietnam," a bitter Irma declares. "The blasts were detonated only when the wind blew in our direction. They avoided the populated areas of Las Vegas and Los Angeles. They saw us as expendable."

Her method of compiling evidence is as simple as it is dramatic. Over the years, she has filled a notebook with the names of 200 acquaintances who developed cancer; whenever one died, she placed a red check next to the name. Today, half of the entries in Thomas' book are so marked. The effects of radiation on the people of St. George are there to see. Marvels one scientist: "Her methods are no different from ours—documentation of numbers. She has shown remarkable civic responsibility and incredible courage—and she has no taint of fanaticism."

Thomas, the mother of four daughters and three sons, had intensely personal reasons for her crusade. Her sister and sister-in-law died of cancer; 15 years ago, her husband Hyrum, 82, developed skin cancer, for which he is still being treated; two of her daughters underwent hysterectomies after pre-cancerous growths were discovered; another suffered three miscarriages. In 1977, after her fourth daughter, a dancer, came down with a severe muscle disease, Irma heard on TV that government officials were denying that radiation had injured St. George residents. "I was so damn mad," she said, "I had to do something."

Thomas started writing letters—to doctors who had examined residents after the atomic tests, to federal officials, to local politicians and to congressmen. Using the Freedom of Information Act, she collected official documents. Searching through the library archives in Salt Lake City and Las Vegas, she hand-copied newspaper accounts of the explosions. Irma even took her case all the way to the White House; after President Carter delivered a human rights speech, she wrote him, "Do we have no human rights?" The letter was referred to a series of bureaucrats, who gave Irma what she calls a "runaround" and only bolstered her determination. "I'm sick of being treated like an imbecile," she says.

The greatest incentive to persevere has probably been the horrifying results of her research. "Thirty people within a one-block radius of my house have gotten cancer," she says. "Ten of them are dead." At first, neighbors were unsympathetic to her fight. "I hesitated about opening old wounds," she recalls. "Many people didn't want to talk about the illnesses in their families. But now I can't get away from it. I go for a walk and people stop me on the street and nail me with their stories."

Some townsfolk believe the tests claimed prominent outside victims, too. During the period of heavy testing, the film The Conqueror was being shot on location in St. George. Its stars, John Wayne and Susan Hayward, and its director, Dick Powell, have all since died of cancer.

While Irma's findings have mobilized the community, they are of course scientifically inconclusive. As one St. George resident puts it, "There's no way to prove that anyone died from radiation." Nonetheless, Irma's hard data has finally attracted high-level interest. Before Senator Kennedy's Health and Scientific Research Subcommittee held hearings in Salt Lake City last spring, two staff members came to consult with Irma. Both of Utah's Senators and Gov. Scott Matheson have promised to investigate, though Irma is skeptical. "I've heard that before," she says. "We'll see." The U.S. Center for Disease Control is conducting a survey of radiation effects in Utah, prompted by a New England Journal of Medicine article linking the abnormally high leukemia death rate among children in the St. George area to the fallout. One scientist who has studied the problem, Dr. Robert C. Pendleton, a radiological ecologist at the University of Utah, gives Irma's work high praise. "Her contribution has been enormous," Pendleton says.

Although many people in St. George are pinning their hopes on the lawsuit, Irma is not a plaintiff. "If I had a stake in the suit, I wouldn't be listened to," she says. "This battle is not for money. I represent hundreds of people who have suffered in silence for nearly 30 years." Her fight is all-consuming: Though she was once a prize-winning potter, Irma says, "I haven't touched a pot since I started raising all this hell." Hyrum Thomas does the shopping and cooking so she can work, and their dining room table is littered with clippings, documents and letters. "What ever will I do with all this mess?" she asks. "I'm wearing down." But she is determined that the government provide testing and treatment for her neighbors who were exposed to radiation. Convinced that any nuclear explosion is dangerous, she also wants to put an end to all underground testing in the U.S. "I just can't stop," she says solemnly. "Something has to be done—while there are still witnesses left."