Should You Worry About Radiation in Japanese Matcha Teas
Most of Japan's tea is grown far to the west of the Fuskushima area, where the 2011 nuclear disaster occurred. The tea producing region in Shizuoka Prefecture is the closest to the disaster area and it is also the only one affected by radiation. Most of the teas tested from the area showed trace amounts of radiation, but did not exceed the safety standards set by the government. A few have been found to contain radiation in excess of the standards, in the dry leaves only; the radiation levels fall far below the safety limits imposed by the government once the tea is steeped.
We must also remember that the radiation limits set by the Japanese government are much more stringent than those of most other organizations, at 500 Becquerels (Bq) per kilogram, which is the same limit adopted by the European Union in 2011. In comparison, the government of the United States has a limit of 1200 Bq/kg, while the limit set by the WHO is 1000 Bq/kg.
In other words, you don't need to worry about tea from Shizuoka Prefecture. Unless you consume your tea leaves directly, you have absolutely nothing to worry about and even if you do, the risk of exposure is minimal. Basically, if you steep your tea leaves, you can safely drink tea from Shizuoka prefecture. Obviously, this means matcha is an exception.
Matcha is a green tea powder is made from whole leaves and is thus consumed whole (more info on matcha green tea). Because it is consumed whole, it would make sense to avoid matcha from Shizuoka, although you are unlikely to be harmed if you do drink some. It is not really much of an issue anyway, as the best matcha comes from the Uji and the Yame regions, neither of which are anywhere near Fukushima.
Any specialty tea shop or online tea shop will be very forthcoming about the origin of their teas. If they are not, I'd suggest finding a new source for your tea. Personally, I don't think there is much to worry about in terms of radiation in tea, but If you are buying matcha or otherwise consume your tea leaves directly or if you simply want to play it very safe, I recommend avoiding tea from Shizuoka and buying from any of Japan's other major tea growing regions instead.