Some plutonium found in soil on the grounds of the Fukushima Daiichi nuclear power plant may have come from its earthquake-damaged reactors, but it poses no human health risk, the plant’s owners reported Monday.

The element was found in soil samples taken March 21-22 from five locations around the plant, the Tokyo Electric Power Company told CNN late Monday. The company said it was equivalent to the amounts that fell on Japan following aboveground nuclear weapons tests by other countries in past decades.

“It is not a health risk to humans,” the company said. But it added, “Just in case, TEPCO will increase the monitoring of the nuclear plant grounds and the surrounding environment.”

Plutonium is a byproduct of nuclear reactions that is also part of the fuel mix at the plant’s No. 3 reactor. It can be a serious health hazard if inhaled or ingested, but external exposure poses little health risk, according to the U.S. Environmental Protection Agency.

Three plutonium isotopes — Pu-238, -239 and -240 — were found in soil at five different points inside the plant grounds, Tokyo Electric reported. It said that plutonium found in two of the samples could have come out of the reactors that were damaged by the March 11 earthquake and tsunami that ravaged northern Japan.

All three isotopes have long half-lives, with plutonium-239 taking 24,000 years to lose half its radioactivity. Plutonium-238 has an 87-year half-life, while plutonium-240’s is more than 6,500 years.

The company said the discovery would not change efforts to bring an end to the crisis at the plant, where three of the six reactors are believed to have suffered damage to their radioactive cores.

– CNN