That is the air radiation. NHK News says the number happens to be much higher than the current air radiation in Iitate-mura (2.1 microsieverts/hour at the village office) in Fukushima Prefecture, where all the villagers have had to evacuate.
Just like in Yokohama City, a citizen measured the air radiation, and alerted the municipal government who then went and measured. At least the municipal governments have started to at least respond.
The result was 2.8 microsieverts/hour. The Setagaya-ku government power-washed the 10-meter stretch of the side walk, and the radiation came down to ... (hold your breath)... 2.71 microsieverts/hour.
Setagaya-ku is one of the special wards of Tokyo that located at the western end of the 23 special wards on the east Tokyo. It borders Kanagawa Prefecture to the south. (Setagaya-ku is marked in deep purple in the map, where the special wards are marked in light purple.)
From NHK News Japanese (10:58PM JST 10/12/2011):
2.7 microsieverts/hour radiation was detected on a municipal road in Setagaya-ku (special ward) in Tokyo earlier this month, and the Setagaya-ku government is asking people to avoid the location while it considers the decontamination plan.
The high radiation was detected in the 10-meter long, 1-meter wide stretch of the side walk in Tsurumaki in Setagaya-ku. According to the Setagaya-ku government, a resident informed the government on October 3 that there was a location with high radiation. When the Setagaya-ku government measured, the radiation was 2.8 microsieverts/hour maximum, much higher than the surrounding area.
The Setagaya-ku government used the pressure washer to wash the stretch of the side walk, but the radiation level only came down to 2.707 microsieverts/hour. The expert whom the government consulted with said the radiation level there may have become elevated because of the rainwater collecting there.
However, the measurement revealed that the air radiation was higher off the ground than on the ground, and the Setagaya-ku government is planning to investigate further to identify the cause of this unusual result.
This municipal road is a designated school route to an elementary school. Setagaya-ku has put up the road cones to alert people not to enter the area, and is considering decontamination. Setagaya-ku measured the radiation in elementary schools, middle schools, kindergartens and nursery schools in July and August, but there was no high radiation detected at that time. It plans to measure the radiation of 258 parks later this month to ensure safety for small children.
Yoko Saito, head of the Setagaya-ku environmental office said, "This is a school route to an elementary school, and there are a kindergarten and a nursery school nearby. I think there are parents who may worry. We will respond as quickly as possible, such as decontamination, after we consult with experts."
Who wouldn't be worried? From what I have heard from an acquaintance, the majority of the parents. They are more worried if their toddlers can get into a prestigious kindergarten. (She is considering moving to Australia with her children.)
How is Setagaya-ku going to "decontaminate", anyway? Power-washing failed. They may have to remove the side walk altogether, and take down part of the yard that borders the sidewalk. I wonder if they are testing the soil or sediment on the side walk. There is no news or announcement at the Setagaya-ku government website.
Looking at the NHK video clip that accompanies the article, the particular stretch looks no different from any other stretch. It is bordering somebody's yard with plants. According to the Ministry of Education and Science aerial survey map of contamination for Tokyo, there should be hardly any contamination for Setagaya-ku.