The number of days with a UV index of eight or higher in Tsukuba, Ibaraki, increased approximately 2.5-fold between 1990 and 2025 [1].

This trend suggests that atmospheric changes are altering the amount of radiation reaching the ground, potentially increasing health risks associated with sun exposure. While cleaner air is generally a public health victory, it may have the unintended consequence of removing a natural shield against ultraviolet rays.

Data from the observation site in Tsukuba shows that in 1990, there were 27 days per year with a UV index of eight or higher [1]. By 2025, that number rose to about 70 days [1]. A UV index of eight is classified as "very strong" or "extremely strong."

Hideaki Nakajima, a senior researcher at the National Institute for Environmental Studies, said that the increase in ultraviolet radiation may be due to improvements in air quality.

According to Nakajima, reduced levels of atmospheric aerosols, such as PM2.5, have changed how radiation interacts with the atmosphere. Previously, these pollutants absorbed or reflected UV radiation, preventing it from reaching the surface. As air quality improved, this protective effect diminished.

Junna Yamagata, a caster for TBS News, said that the Japan Meteorological Agency provides hourly UV information divided into 13 levels to help the public manage their exposure [2].

The 35-year period of observation highlights a shift in the environmental landscape of Ibaraki Prefecture. The rise from 27 to 70 days [1] indicates a significant change in the frequency of high-risk radiation days for residents and visitors in the region.

“The number of days with a UV index of eight or higher in Tsukuba, Ibaraki, increased approximately 2.5-fold between 1990 and 2025.”

This finding illustrates a complex environmental trade-off: the reduction of industrial air pollutants, while beneficial for respiratory health, removes the 'aerosol masking' effect that once shielded the earth's surface from intense UV radiation. As Japan continues to meet stricter air quality standards, public health strategies may need to shift toward more aggressive skin and eye protection to compensate for the loss of this atmospheric filter.