Tokyo's Narita Airport reports it has been affected by minimal levels of radiation from the badly-damaged Fukushima Daiichi nuclear power station, some 195km to the north.
According to the airport, the radiation dose detected atop the telecommunications tower at the airport was 118 micro sieverts. By way of comparison, says the airport authority, a chest x-ray accounts for 50 micro sieverts.
There are 1,000 micro sieverts in one milli sievert and the annual exposure calculation for someone at Narita would be just over 1 milli sievert. According to the World Health Organisation, a person is typically exposed to natural background radiation of around three milli sieverts over the course of a year.
According to the Airports Council international, a UN agency task force has assessed that current radiation levels are well within safe limits and do not pose a hazard for people travelling to Japan.