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What's behind the foul mystery odor around Yokohama?

Four cities which have received reports of strange smells. (Mainichi)

YOKOHAMA -- Since June, there have been repeated complaints of a strange smell of unknown origin in the city of Yokohama and other areas of the Miura Peninsula in Kanagawa Prefecture, south of Tokyo.

    Many residents have described it as "like burned rubber," or "similar to gas," but the source of the odor remains a mystery. A number of possible causes are being posited, and component analysis by the Kanagawa Prefectural Government has strengthened the hypothesis that the source is not of natural origin.

    It started on the night of June 4, when the first reports came in from residents of the city of Miura's Minamishitaura-machi neighborhood. In what seemed to be a pattern stretching north along the peninsula's eastern coast up to the Oppama district of the city of Yokosuka, some 500 reports were filed to local police and fire departments. One woman in her 40s in Yokosuka was even transported to hospital for feeling unwell, though her symptoms were minor.

    The Miura Peninsula subsequently saw brief concentrations of reports of a strange odor at a pace of once a month. On Oct. 1, the smell had spread further north to Yokohama. On the evening of Oct. 12, passengers at JR Yokohama Station also complained of a stink, and the station's ticket barriers at the central south gate were closed for some 20 minutes.

    As of Oct. 15, there had been nine commotions around foul smells in four months in the prefecture, with around 680 reports. According to the Yokosuka city fire department, in an average year they see about one instance of complaints about strange odors per summer. A department employee commented, "We've never had reports coming in at this rate before."

    The prefectural government and Yokohama Municipal Government are now investigating the phenomenon. On Oct. 12, when the municipality got 16 reports of foul odors, public employees at the city fire department's headquarters in Hodogaya Ward smelt it too, and specialist equipment was used to take air samples.

    Analysis by the Yokohama Environmental Research Institute found high concentrations of isopentane, pentane and butane, which are released when gasoline and other fuels vaporize, among other substances. The city government said they do not present an immediate heath threat.

    Additionally, samples taken in Yokosuka by the prefectural government on Oct. 14 were announced on Oct. 16 to contain isopentane and other substances. Although the same components were found in the Yokohama samples, the Yokosuka ones had lower concentrations.

    The central government is also responding to the issue. Minister of the Environment Shinjiro Koizumi, whose parliamentary constituency includes Yokosuka and Miura, said at an Oct. 13 press conference, "This is an issue unlike any we've seen before, so I'm hearing even inside the ministry that it's a difficult case. We want to help get to the bottom of it." At an Oct. 14 press conference, Chief Cabinet Secretary Katsunobu Kato also said, "The central government will use its capacity to assist investigations into the source of the odor and what it is composed of."

    But what are the causes under consideration? Makoto Wada, professor emeritus at Tokushima University and a specialist in organic chemistry, cited the city and prefectural governments' discovery of large amounts of gasoline substances in its analysis to suggest that car exhaust fumes, marine gas tanker venting or gas leaks from petrochemical plants and oil tanks may be to blame.

    Wada also said, "The substances are included in fuels, and many of them exist around us too, so it's probably quite difficult to ascertain exactly what's there. It's also possible that substances the current analysis measures can't detect are the cause."

    Kohei Urano, professor emeritus in environment safety management at Yokohama National University, told the Mainichi Shimbun that he thinks the descriptions of gas and burnt rubber smells in the residents' reports suggest sulfur compounds leaking from petroleum refineries could be the culprit. He speculated, "It could be the sulfur compounds emitted when rubber products are burned. They're also used as odorants for gas."

    Urano also said it could be coming from the oil refineries on the coasts of Yokohama and Yokosuka. Regarding the constituents in the analysis results, he said, "Unlike substances like isopentane, even small amounts of sulfur compounds emit perceptible odors."

    Tokai University professor Yoshihiko Yamada, who specializes in ocean policy, says a blue tide could be behind the smell. Blue tide is a phenomenon in which dead plankton that collects at the bottom of the ocean breaks down and oxygen levels reduce, causing water containing sulfide to rise to the surface, which in turn makes the ocean look pale blue. The hydrogen sulfide produced in blue tides are said to smell like rotten eggs or gasoline.

    Yamada said, "The seafloor in the Tokyo Bay area around Yokohama and Yokosuka has depressions in it from when they've been dredged. Furthermore, with this summer bringing fiercely hot temperatures, the sea water temperature has been high, making it easier for plankton to gather at the bottom. It's become an environment which accommodates blue tide events."

    He also cited the fact the odors do not linger for long as a supporting factor, saying, "Blue tides quickly disappear, and the fact that the smell also disappears quickly stacked up for me." But Yamada did say that isopentane and other substances found in the sample analyses are not produced by blue tides.

    Yoichi Hasegawa, the head of the prefectural government's air and water quality department which is handling the investigation, told the Mainichi, "There appear to be certain commonalities (between the sample analyses). We can't absolutely rule out a natural cause, but we think it probably came from a human-caused factor at a facility. Oil-related substances are everywhere, so it's difficult to ascertain a specific origin."

    (Japanese original by Mami Miyajima, Shotaro Kinoshita and Tsumuki Nakamura, Yokohama Bureau)

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