"In an exercise of caution and just to make sure everyone remains safe - we are doing screening of passengers and/or cargo if there happens to be even a blip in terms of radiation. We have seen no radiation by the way even on incoming cargo or passengers that comes close to reaching a harmful level," said Homeland Security Director Janet Napolitano.
Besides monitoring passengers, cargo and mail coming from Japan are also being checked. Meanwhile, there are a lot more passengers' families leaving Japan and arriving at SFO because of the risk.
U.S. Customs and Border Protection says personal radiation detectors are being used at airports. SFO has sophisticated equipment to also determine the type of radiation. Passengers may not even be aware they are being monitored. The threat of being exposed worried Joan Sarlatte. She had to make a connecting flight through Tokyo.
"As we heard more explosions, we obviously became more concern, but the Delta folks just kept saying, 'We are checking, things are good, we are still going through Narita, don't worry,'" said Sarlatte.
On Thursday, it looked like there was an increase of families who left Japan fearing for their health. Finding a flight is nearly impossible. Ann Yamamoto lives in Tokyo and ended up leaving from Osaka -- a seven hour drive. She says the immigration offices were even harder to deal with.
"The immigration area was jammed packed with people who were leaving on emergency because they didn't have the proper visa to get out of the country," said Yamamoto.
Hannah Breslau is a high school freshman. She was studying at the International school in Kobe. She admits being concerned.
"A little bit. There was a lot of hype on the news about the nuclear reactor, about the plants and explosions and it was kind of scary," said Hannah.
Her father says it took some work getting her out of Japan.
"There's no specific danger that she was in directly, but just uncertainty about what is going to happen over the next few days and weeks," said Joshua Breslau, Hannah's father.
Yamamoto says it was her family who insisted she travel to San Francisco.
"When everything is stabilized, we plan to go back," said Yamamoto.
Some at SFO, arriving from Japan, say they feel somewhat guilty leaving loved ones behind, but they say their children in this case come first.
U.C. Berkeley has suspended its study aboard programs in Japan. This comes one day after the CSU system ordered all of its students studying in Japan to return home. Cal's program in Japan includes about 80 students and 30 staff members. CSU has 45 students in Japan. All of the students are safe and have been accounted for.
ABC7's Janelle Wang contributed to this report