The Japanese nuclear power plant crisis is triggering jitters about radioactive fallout hitting the United States, even though authorities say it is highly unlikely significant amounts of dangerous material will travel across the Pacific Ocean.

Fearful residents have flooded health officials in western states such as California, Washington and Oregon with anxious questions, and some authorities have begun issuing updates about air monitoring for radiation.

“We opened a hotline and have fielded hundreds of calls from the worried public,” said Michael Sicilia of the California Department of Public Health.

The two U.S. companies that make potassium iodide, which can reduce the risk of thyroid cancer from exposure to iodine-131, are being overwhelmed by demands for the medication from individuals, pharmacies, hospitals, day-care centers and others.

“People are terrified,” said Alan Morris, president of Anbex Inc., of Williamsburg, Va. “We’re getting calls from people who are crying and saying things like, ‘Please. Can’t you help me? Can’t you send me anything?’ ”

Both companies, along with state and federal officials and independent radiation experts, have been trying to reassure people that the chances of dangerous amounts of radiation reaching the United States from Japan are negligible, making such precautions unnecessary.

“All of the information available right now indicates there will be no harmful levels of radioactivity in the United States,” said Scott Burnell, a spokesman for the Nuclear Regulatory Commission. “There’s absolutely no reason for concern.”

Nevertheless, as a precaution, the Environmental Protection Agency sent seven mobile air monitoring stations to Hawaii, Alaska and Guam to bolster capabilities to detect any radiation from Japan. The agency already has more than 100 permanent air monitoring stations around the United States, including in Alaska and Hawaii, but decided to deploy the additional equipment to heighten the early-warning system.

In the meantime, thousands of people are seeking potassium iodide. CVS’s online pharmacy sold out of it over the weekend, a spokesperson said.

“I’m very concerned,” said Laurie Akey, 58, of Newport Beach, Calif. After studying news reports and weather patterns, Akey ordered enough potassium iodide for her husband, four children, three grandchildren, grandparents and Lizzy, her 5-year-old King Charles Spaniel. “This thing is blowing apart over there. If this thing keeps blowing it could come over in a cloud and land on our shores.”

Available without a prescription, potassium iodide blocks radioactive iodine from accumulating in the thyroid gland, where it would boost the risk for thyroid cancer. Thousands of cases of thyroid cancer occurred after the Chernobyl disaster, primarily among those who were children at the time and drank milk from contaminated cows.

People who think they might be exposed to radioactive iodine can start taking doses as soon as they fear they may be at risk for exposure. Potassium iodide is generally safe, although it can pose risks to people who are allergic to iodine and shellfish or have certain skin or other disorders, and can cause heart problems, nausea, vomiting and bleeding.

“It’s been a frenzy — that’s the only thing I can use to describe it,” said Deborah Fleming Wurdack, co-owner of Fleming Pharmaceuticals of St. Louis, which sells a liquid version of the drug in a bottle with a dropper for $13.25 containing 135 adult doses that can be used by adults, split up for teenagers and children of any age.

The small privately owned company has been “fielding hundreds of calls. People are showing up at the door. We’ve heard from Singapore. We’ve heard from Japan. We’ve heard from Korea. We’ve heard from states ordering large quantities,” Fleming said. “We’ve heard from mothers wanting to get it in their house to protect their children. We’ve heard from pharmacies, hospitals, day care centers. It’s just been constant.”

The company expected to have exhausted its supply of 50,000 bottles by the end of the day Wednesday, but is gearing up to resume production, hiring temporary workers to answer the phone and upgrading its Web site to let people order it directly with a credit card, she said.

“It’s crazy,” she said. “Some of these people are in a panic mode. The saddest call we got was from a quadriplegic in California who wanted to protect his children because he can’t get away from the plume if the plume is coming to California.”

Anbex, which sells bottles of 14 tablets for $10 that adults can take, estimates they have been getting two or three calls a minute for days. The company exhausted its stock of about 10,000 blister packs by Friday night. The company has resumed producing the drug and expects new supplies by the end of March or beginning of April.

“It’s been unbelievable. I think I’ve spoken to three quarters of the population of California. We are trying to tamp down the feeling that the world is falling apart. It’s not falling apart,” Morris said.

Meanwhile, Rep. Edward Markey (D-Mass.) has asked federal officials to provide potassium iodide, also known as KI, to everyone living within 20 miles of a nuclear power plant in the United States. Currently, the drug is made available to everyone within 10 miles.

“We should not wait for a catastrophic accident at or a terrorist attack on a nuclear reactor in this country to occur to implement this common-sense emergency preparedness measure,” Markey wrote to John Holdren, director of the White House Office of Science and Technology Policy.

In response, Dori Salcido, a spokeswoman for the Health and Human Services Department, said the “U.S. government will be studying every aspect of the Japanese disaster and the Japanese government’s response, with the goal of learning as much as possible from that review. Policy options relating to KI distribution will be among the issues studied.”

Aside from taking potassium iodide, people can reduce their risk from radioactive fall-out by staying inside, covering their mouths and noses if they are outside in a contaminated area, immediately washing off any exposed clothes or body parts with soap and water, and avoiding ingesting anything tainted. In some cases, hot areas are evacuated.

But officials stressed that there is no indication that any of those precautions will become necessary in the United States.

“The public does need to pay attention. If the public receives specific instructions from their local government, all of these approaches will be based on the best information available,” Burnell said. “But at this time there’s no indication that anyone in the United States is going to see harmful levels of radiation.”

The only other treatment for radiation exposure is a drug known as Prussian Blue, which binds to cesium so the body can eliminate it through the digestive system.

People exposed to high levels of radioactive material, which is usually confined to first-responders and workers at damaged plants, may need intensive medical care.

“Prevention is really the key,” said Fred A. Mettler Jr., a radiation expert and physician at the University of New Mexico. “Whatever us doctors can do afterwards is pretty limited.”

Mettler noted, however, that other sources of iodine are available if potassium iodide is not. Seaweed, for example, is rich in iodine.

“You can just go to your local sushi place and order some seaweed and eat it,” Mettler said. “But there really isn’t any need for that in this country.”