Poison gas may have been abandoned at 138 sites in 41 prefectures at the end of World War II, according to the results of a nationwide study released Friday by the Environment Ministry.

"The government will consider appropriate measures to deal with (abandoned poison gas) as early as next month," Environment Minister Yuriko Koike told a regular news conference in response to the study results.

The ministry launched the probe in June, after a series of incidents in which health problems were linked to abandoned poison gas came to light.

Although internal documents on a 1973 study on the matter were found in May, the Environment Ministry felt the need to collect more information.

The 138 cases were placed into four categories in accordance with the credibility of the information collected about the site in question. Four sites -- including those where people have already been sickened -- have been classified as priority sites requiring measures aimed at ensuring safety.

The four include a site in the city of Hiratsuka and the town of Samukawa, both in Kanagawa Prefecture, one in the town of Kamisu, Ibaraki Prefecture, and one in the city of Narashino, Chiba Prefecture.

Information that 16 other sites were contaminated was meanwhile deemed reliable, though this information failed to specify exactly where the poison gas was dumped. These sites were thus placed in another category.

A further 21 sites were pinpointed, although ministry officials determined the certitude of the collated information was low.

For these 37 areas, the ministry said it would continue to collect information and monitor the situation.

The study also found that 34 other sites, scattered from Hokkaido to Oita Prefecture, hosted facilities that produced and stored poison gas.

It also identified 44 sites where chemical weapons were reportedly abandoned and 823 reported injuries related to poison gas.

Akita, Yamanashi, Gifu, Mie, Tottori and Shimane were the only prefectures that were not linked to reports regarding the production or abandonment of poison gas weapons, according to the ministry.

In conducting the study, the ministry sought information related to poison gas from other ministries, as well as from municipal governments. It also asked citizens to come forward with any information regarding the use and disposal of chemical weapons.

In September 2002, construction workers in Samukawa fell ill after coming into contact with bottles containing poison gas residue, while workers at a construction site in Hiratsuka reported health problems in April. Soil at the site was later found to be contaminated by substances related to chemical weapons.

In March, Kamisu residents complained of illness after drinking well water contaminated by arsenic. The latter is believed to have emanated from poison gas abandoned by the Japanese military.

Yet a ministry study involving intensive drilling and radar failed to unearth concrete evidence proving a link between the water and chemical weapons.

Despite prior expectations, the latest survey failed to offer useful information about the link between poison gas and water contamination in Kamisu.