While the government is cautious about placing blame, experts say much of the rise in pollution is coming from China, where air quality is a focus ahead of the Beijing Olympics next year.
The type of smog — called "photochemical smog" because it is created when sunlight reacts with exhaust from cars and factories — is made up of photochemical oxidant particles such as ozone. These particles can cause breathing difficulties and headaches.
"In terms of average levels of photochemical oxidants measured annually across Japan, there has been quite a rise since the 1990s," said Toshimasa Ohara, head of the National Institute of Environmental Studies’ regional atmospheric modeling section. "We believe a substantial part of that rise has come from increasing emissions in China. We’re looking into what percentage this factor has accounted for."
Smog adds to a string of environmental concerns that experts say originate in China, including acid rain and sandstorms that gain toxicity as they pass over its industrial regions.
But academics say Japan may find it hard to put pressure on China to cut emissions, with studies yet to show a precise figure on how much of Japan’s smog is caused by cross-border pollution.
"If we are going to take action against other countries, we can’t be vague," said Atsuko Mori, senior researcher at the Institute for Environmental Research and Public Health in Nagasaki, southern Japan. "There needs to be a thorough, scientific study into the causes."
Mori and other experts say research is complicated because domestic factors are also to blame for the recent rise in smog across Japan, which has taken pride in its efforts to cut emissions since its days of rapid economic growth in the 1970s.
For example, while emissions from cars have been restricted, those from paint and gasoline vapours, which also contribute to smog, have been harder to control. Smog can also be exacerbated by strong sunlight.
The Environment Ministry asked a group of academics and local health officials last month to carry out a study on pollution trends, but detailed research into the causes could take years.
"Research to base environmental policies on requires a lot of time and money," said Hajime Akimoto, programme director at the Japan Agency for Marine-Earth Science and Technology, noting that the United States spent a decade on research before it took steps against cross-border pollution.
"Research like that in Japan could take another five years."
As a first step, government officials say Japan is working together with China to measure its pollution, although the country still lacks high-tech equipment to analyse some pollutants such as ozone.
Ohara at the National Institute of Environmental Studies said the region could in future look to the example of the Convention on Long-Range Transboundary Air Pollution, an agreement to cut pollution under the UN Economic Commission for Europe signed by countries such as the United States and Canada.
"If it becomes clear that the effects of cross-border pollution are big, then it will be imperative to create international regulatory rules within East Asia, similar to Europe," he said.