India's nuclear authorities have admitted radioactive scrap was exported from the country to make lift buttons in France.

French firm Mafelec sent thousands of lift buttons to the elevator maker Otis, which put them in hundreds of lifts in the country over the summer. Otis said it removed the buttons after France's nuclear safety authority (ASN) announced this week that 20 workers who handled them were exposed to doses of radioactivity ranging from 1 to 3 millisievert (mSv). The French legal limit for people who do not work in the nuclear industry is 1mSv per year.

The ASN said it had classified the incident at Mafelec as level two on the international nuclear event scale. The scale goes from zero, which means no safety risk, to seven, which means a major accident.

The safety agency said the buttons contained traces of radioactive cobalt-60. Four Indian firms made the components, an Indian official said. It was unclear where the contaminated scrap originated – although metal was traced to a foundry in the western state of Maharashtra. Malafec bought the buttons from two Indian companies that got their raw materials from another company, SKM Steels. In turn, it was supplied by the Vipris foundry near Khopoli on the way to Pune from Mumbai. "We are tracking back the whole chain," said Satya Pal Agarwal, the head of the radiological safety division of India's atomic energy regulator.

Indian foundries are not required to install radiation detectors to check scrap, but the government has a programme to put radiation monitors at ports to check cargo.