Customs and health officials began taking the temperatures of passengers arriving at New York’s Kennedy International Airport from three West African countries on Saturday in a stepped-up screening effort meant to prevent the spread of the Ebola virus.

Federal health officials said the entry screenings, which will expand to four additional U.S. airports in the next week, add another layer of protection to halt the spread of a disease that has killed more than 4,000 people.

“Already there are 100 percent of the travelers leaving the three infected countries are being screened on exit. Sometimes multiple times temperatures are checked along that process,” Dr. Martin Cetron, director of the Division of Global Migration and Quarantine for the federal Centers of Disease Control and Prevention, said at a briefing at Kennedy.

Cetron added, “No matter how many procedures are put into place, we can’t get the risk to zero.”

The screening will be expanded over the next week to New Jersey’s Newark Liberty, Washington Dulles, Chicago O’Hare and Hartsfield-Jackson in Atlanta.

Customs officials say about 150 people travel daily from or through Liberia, Sierra Leone or Guinea to the United States, and nearly 95 percent of them land first at one of the five airports.

Passengers arriving at Kennedy said the process of taking people aside for screening was orderly.

“They asked us a few questions, if we had been sick in the past few days,” said 14-year-old Johnson Nellon, who flew from Liberia with his 17-year-old brother.

One man said his temperature was taken even though his country, Niger, does not have an Ebola outbreak. “They checked everything and everything was fine,” Moussa Halidou said.

Public health workers use no-touch thermometers to take the temperatures of the travelers from the three Ebola-ravaged countries; those who have a fever will be interviewed to determine whether they may have had contact with someone infected with Ebola. There are quarantine areas at each of the five airports that can be used if necessary.

There are no direct flights to the U.S. from the three countries, but Homeland Security officials said last week they can track passengers back to where their trips began, even if they make several stops. Airlines from Morocco, France and Belgium are still flying in and out of West Africa.

A traveler from Nigeria, which has been lauded for its successful effort to contain Ebola, said she was screened twice in her native country: once in Lagos and again in the capital, Abuja, where she had a connecting flight.

“Nobody’s panicking in Nigeria right now,” the woman, Nonye Ike, said. “Everybody that has any fever or something, you’ll just be isolated. I’m so grateful.”

President Barack Obama has said the new screening measures are “really just belt and suspenders” to support protections already in place. Border Patrol agents already look for people who are obviously ill, as do flight crews.

Health officials expect false alarms from travelers who have fever from other illnesses. Ebola isn’t contagious until symptoms begin, and it spreads through direct contact with the bodily fluids of patients.

Cetron said more than 36,000 travelers leaving West Africa have been screened for Ebola in the last two months and none was infected with Ebola.

The extra screening at U.S. airports probably wouldn’t have identified Thomas Eric Duncan when he arrived from Liberia last month because he had no symptoms while traveling. Duncan, the first person to be diagnosed with Ebola in the U.S., died Wednesday in Dallas.

Experts say the federal government has broad authority to screen passengers and quarantine them if necessary.

The CDC cited as legal authority the Commerce Clause of the Constitution, under which the government regulates trade with foreign countries. The 1944 Public Health Service Act also allows the federal government to take action to prevent communicable diseases, which include viral hemorrhagic fevers such as Ebola, from spreading into the country.

(AP)