Sales of nuclear shelters and radiation-blocking air purifiers have surged in Japan in recent weeks as North Korea has pressed ahead with missile tests in defiance of U.N. sanctions.

A small company that specializes in building nuclear shelters, generally under people’s houses, has received eight orders in April alone, compared with six orders during a typical year.

The company, Oribe Seiki Seisakusho, based in Kobe, has also sold out of 50 Swiss-made air purifiers, which are designed to keep out radiation and poisonous gas, and is trying to get more, said Nobuko Oribe, the company’s director.

A purifier designed for six people sells for ¥620,000 and one designed for 13 people and usually installed in a family-use shelter costs ¥1.7 million.

Concerns about a possible gas attack have grown after Prime Minister Shinzo Abe told the Diet this month that North Korea may have the capacity to deliver missiles equipped with sarin nerve gas.

“It takes time and money to build a shelter. But all we hear these days, in this tense atmosphere, is that they want one now,” Oribe said. “They ask us to come right away and give them an estimate.”

Another small company, Earth Shift, based in Shizuoka Prefecture, has seen a tenfold increase in inquiries and quotes for its underground shelters, according to Akira Shiga, a sales manager at the company.

The inquiries began to increase gradually in February and have come from all over Japan, he said.

North Korean missiles have been fired with increasing frequency. Last month, three fell into waters within Japan’s exclusive economic zone, some 300-350 km off the coast of Akita Prefecture.

The central government on Friday urged local governments to hold evacuation drills in case of a possible missile attack, heightening a sense of urgency among the public.

Some orders for the shelters were placed by small business owners for their employees, and others by families, Oribe said. A nuclear shelter for up to 13 people costs about ¥25 million and takes about four months to build, he said.

The shelter his company offers is a reinforced, air-tight basement with an air purifier that can block radiation as well as poisonous gas. The room is designed to withstand a blast even if a Hiroshima-class nuclear bomb exploded just 660 meters away, Oribe said.

North Korea said Sunday it is ready to sink a U.S. aircraft carrier to demonstrate its military might, in the latest sign of rising tensions.

The United States ordered the USS Carl Vinson carrier strike group to sail to waters off the Korean Peninsula in response to mounting concern over the reclusive state’s nuclear and missile programs.

In Japan’s previous experience with sarin gas, the Aum Shinrikyo doomsday cult killed 12 people and made thousands ill in attacks on Tokyo subways in 1995.