The US Customs and Border Protection (CBP) agency states no aircraft entering the country from Japan have tested positive for radiation at harmful levels.
CBP frontline personnel use personal radiation detectors to detect radiological materials and all airports have radiation isotope identification devices (RIIDs) to determine presence and type of radiation encountered, the agency states.
"Out of an abundance of caution, CBP has issued field guidance reiterating its operational protocols and directing field personnel to specifically monitor maritime and air traffic from Japan," says the CBP.
The caution was triggered by the loss of cooling functions lost in a number of reactors Fukushima Daiichi nuclear plant after the 11 March earthquake and ensuing Tsunami that struck the Northern part of the country.
Airlines have also bolstered their radiation monitoring. Austrian Airlines on 15 March sent an expert to Tokyo Narita to measure radioactivity levels to determine if it is safe to keep operating to Japan, while Austrian parent Lufthansa on 13 March took a precautionary measure and began examining radioactivity levels on aircraft returning to Germany from Japan.