SAN DIEGO -- Small amounts of radioactive material from the crippled Japanese nuclear power plant are being blown toward San Diego over the Pacific Ocean and will reach the coast as soon as Friday, but there is no need for Southern California residents to take any special precautions, county officials said.

The county is working closely with national and state officials to monitor the radioactive plume emanating from the nuclear reactors crippled by the earthquake and tsunami in Japan last week, according to San Diego County's Health and Human Services Agency.

Although a United Nations forecast obtained by The New York Times showed the plume would reach Southern California on Friday, the level of radiation is not expected to be hazardous, and there's no reason to take iodine tablets, said Dr. Eric McDonald, the county's deputy public health officer.

"Potassium iodide is used to protect the body from one specific type of radioactive material known as radioiodine,'' McDonald said. "Potassium iodide should only be used in cases of exposure to significant amounts of radioiodine.''

Thursday, President Barack Obama also said that Americans need not take special precautions beyond staying informed.

"We do not expect harmful levels of radiation to reach the United States," the president said.

The California Department of Public Health and the state Emergency Management Agency issued a similar analysis Thursday.

"We want to emphasize that the U.S. Environmental Protection Agency, the Nuclear Regulatory Commission and the U.S. Department of Health and Human Services have all stated that there is no risk expected to California or its residents as a result of the situation in Japan," the agencies said in a news release. "We urge Californians to not take potassium iodide as a precautionary measure. It is not necessary given the current circumstances in Japan .. and taken inappropriately it can have serious side effects including abnormal heart rhythms, nausea, vomiting, electrolyte abnormalities and bleeding."

The Environmental Protection Agency is closely watching radiation levels with a system of 100 radiation monitors, spread across the country in places such as Anaheim, Bakersfield and Eureka. In addition, a network of 63 sensors is operated by the Comprehensive Test Ban Treaty Organization, an international agency allied with the United Nations.

Atmospheric experts said the material should begin showing up on the West Coast as early as Friday, though it could take up to an additional week for the 5,000-mile trip from Japan to Southern California. Although the organization has told its member countries that the first indication of radiation would hit on Friday, the plume from a North Korean nuclear test in 2006 took about two weeks to travel to North America, U.N. officials said.

The Nuclear Regulatory Commission, which oversees the U.S. nuclear industry, said Wednesday that it did not expect dangerous levels of radioactivity to hit the West Coast, Hawaii, Alaska or U.S. territories in the Pacific. But whatever levels reach the U.S. initially are likely to increase in subsequent days, because radioactive emissions from the damaged Fukushima Daiichi nuclear power plant have grown since the disaster began Friday.

Edwin Lyman, a specialist at the nuclear watchdog group Union of Concerned Scientists, said that although it was true that the more radioactivity released in Japan the more could migrate away from the region, he did not think the U.S. was at serious risk.

 "We can never say never," Lyman said. "My judgment is that there will probably be measurable radiation, but except for a few hot spots it is not something we should really worry about."