A study by Japan's Meteorological Research Unit determined that the most recent wave of PM2.5 reported in central parts of the country and around Tokyo were in fact due to volcanic gas carrying sulphur dioxide from Mount Sakurajima in southern Kyushu. PM2.5 are the tiny airborne particles considered most harmful to health.
The volcano, which towers above the city of Kagoshima, has been classified as active since 1955 and hundreds of minor eruptions every year have ensured that a steady stream of ash is spewed out.
Activity increased earlier this year until a major eruption on August 18 saw a plume rising five kilometres above the crater and significant amounts of ash falling on surrounding towns.
The agency has conducted a simulation of the path of the winds and the ash and determined that falls of PM2.5 particulate matter in Osaka, Tokyo and along the southern coast of Japan are from the volcano. Satellite data from Nasa has backed up those findings.
In the first week of November, officials in Chiba prefecture, to the east of Tokyo, told residents to stay indoors because the average hourly density of PM2.5 had exceeded the limit of 70 micrograms set by the government.
Local media warned the public to be prepared for further inundations of pollution as Chinese power stations start burning large amounts of coal for heating during the coming winter months.