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Air Pollution in Japan - Is It Getting Worse?

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What's the air pollution in Japan like during:

Air pollution in Japan during Winter

During the winter, air pollution in Japan is particularly bad since the polluted air comes not only from industrial and vehicle emissions within Japan, but also from polluted air masses that are derived from China.

Smog over Tokyo

During the winter, cold, dry air moves in from China as well as from Siberia and passes over the warm and humid Sea of Japan. As a consequence, the western side of Japan receives much higher precipitation (i.e. snowfall) totals than the eastern coastline.

Furthermore, the majority of the western side of Japan receives large amounts of wet deposition acidic compounds (i.e. acidic precipitation). In fact, the worst region in Japan for acidic precipitation occurs along the central western coastline around the Toyoma and Niigata Prefectures.

A number of scientific studies have shown that a lot of the forests situated along the western facing mountain slopes 2000 metres above sea-level have been severely damaged by the high amount of acidic snowfall. In comparison, the forests located near sea-level are still quite healthy since the precipitation is more alkaline. Unfortunately, if China doesn't begin to reduce its smog emissions from its industrial regions, then the air pollution in Japan (particularly the western part) during the winter time will become worse in the future. Of course, this will lead to a further decline in the health of the western Japanese forests.

Air pollution on the eastern side of Japan is considerably lower during the winter than the western side. Overall, the eastern part of Japan is known to have much cleaner snowfalls. However, the snow in the eastern part of Japan can be polluted if it falls in an industrial or highly urbanised area like the Kanto Plain district.

Air pollution in Japan becomes problematic on cold, clear still nights. After the sun sets, many air pollutants begin to accumulate near the ground surface since there's no wind to disperse them. This is particularly true for locations that are located in a valley. These air pollutants build up to very high levels during the night and as such, it can be quite harmful for anyone (irrespective of whether you're predisposed to respiratory problems or not) to go walking outside in such smoggy conditions. Normally, the air pollutants are dispersed once the sun begins to rise.

Air pollution in Japan during Spring

In Spring, the wind regime in Japan becomes less dominated by air masses blown in from China and thus air pollution in Japan begins to drop in most areas, except in the Kanto Plain district.

However, you need to be aware that pollen levels increase during March and this leads to higher levels of air pollution in Japan, particularly in industrial and urbanised areas.

During springtime, the weather warms up quickly with each passing day and by the end of Spring, it becomes quite hot and humid. Spring brings with it Kosa events (i.e. dust storms that blow in from the Northern China deserts). These dust storms impact central and northern parts of Japan. Around this time, beginning in April, haze is frequently observed over northern Japan due to the Boreal forest fires which burn profusely in Siberia and the Russian Far East.

These dusts storms blown in from deserts of northern China contain high amounts of calcium (Ca2+), which are derived from the calcium carbonate (CaCo3) dust particles. The CaCo3 is responsible for neutralising acidic precipitation over Japan during the spring time and thus is another reason why acidic precipitation is generally lower during spring than any other time of the year.

Air pollution in Japan during Summer

Morning smog over Osaka in summerThere is normally low to medium levels of air pollution in Japan in Summer. However, if there's no wind, you're likely to get a build up of smog if you're located in a valley.

Although, air pollutants can be very high in summer if a typhoon (same thing as a hurricane) impacts southern Japan. The air pollutants from China are drawn into the main typhoon circulation and become integrated into the precipitation. This leads to widespread acidic rain over Japan during such events. However, when there's no typhoon or any other low pressure system near Japan in summer, air pollution levels are normally quite low except in cities or regions that high levels of vehicles or industrial emissions.

Since the weather in Japan in summer is quite hot and sunny, most of the hydrocarbons emitted by vehicles are converted to ozone when they're exposed to intense sunlight. For this reason, you can expect that surface ozone levels are highest during the summer. Conversely, surface ozone are lowest during the overcast winter months.

Ozone is a potent oxidant and is known to cause major irritation to most people by aggravating your eyes and respiratory system. It also causes much damage to plants that grow in urban areas of Japan. Furthermore, many people may not realise this but ozone readily attacks rubber. That's why windscreen wiper blades and vehicle tyres wear out more quickly in highly polluted areas.

This surface ozone should not be confused with the term 'ozone layer'. If you suffer from respiratory problems such as asthma, then you should avoid visiting or living along roads with heavy traffic in Japan so as to minimise your exposure to both the vehicle fumes (carbon monoxide and nitrogen oxides) as well as the photochemical oxidants (ozone) derived from this.

During the summer, sea breezes are very common along the coast of Japan. If you happen to be staying in a large city near the coast, you will notice when you look out over the sea (if your view is not blocked) in the morning that smog is lying out to sea. When the sea breeze arrives from midday onwards, this pushes the smog towards the coast and over the city. The smog is then blown back out to sea when the land breeze kicks in at around 10pm during the balmy summer nights.

This recycling of air pollutants by the sea breeze is quite common, not only in Japan, but along all urbanised coastlines of mid-latitude and tropical regions of the world during the summer months.

However, the most severe air pollution in Japan during the summer at sea-level occurs in the region known as the Kanto Plain (as I've already mentioned earlier).

Air pollution in Japan during Autumn

Peak hour traffic in Osaka

During autumn (fall), air pollution in Japan begins to increase as winter approaches. Sea breezes become less common with each passing week in autumn.

The atmosphere at night becomes a little more stable and this acts to trap air pollutants emitted by both industrial and vehicle emissions close to the ground, thus air pollution concentrations tend to rise higher at night than during the daytime.

Pollinosis and asthma is a major problem in Japan

The main cause of pollinosis and asthma related diseases in Japan is from the Japanese cedar trees. These trees are common throughout the forests in mountainous areas of Japan. The pollen it produces is particularly small and has a low density. These two factors combined with the fact that this pollen is highly allergenic and can float around in the air for a considerably longer time than most other pollens makes the pollen particularly worrisome.

The Japanese cedar pollen season runs from February to the beginning of April, with the highest airborne cedar pollen levels being recorded in March each year. If you know without a doubt that you suffer from pollinosis or asthma, then make sure you don't visit the Kanto Plain district in Japan during the Japanese cedar pollen season. But why, you ask...?

The Kanto Plain is known as the largest source of air pollution in Japan irrespective of whether it's summer, winter, spring or autumn (fall). The Kanto Plain covers a large area and includes Tokyo metro as well as the Prefectures of Kanagawa, Chiba, Saitama and Ibaraki.

During the Japanese cedar pollen season large amounts of pollen grains travels into polluted urban areas. When this occurs, most types of air pollutants (heavy metals and acidic substances) quickly attach themselves to the pollen grains' surface and get a free ride across the city. Of course, the pollen itself doesn't remain unaffected by its new host. Most polluted pollen grains become highly acidic.

As a result, if you inhale any of these heavily polluted Japanese cedar pollen grains, you'll greatly increase the chance that you'll suffer an asthma attack or pollinosis. If you know without a doubt that you suffer from any respiratory illnesses, then try and avoid visiting Japan during the Japanese cedar pollen season and in winter.

There are two locations that either have very little or no pollen floating through the air since Japanese cedar trees have never grown here since ancient times:
  1. Hokkaido - the most northern island in Japan
  2. Okinawa - is the Prefecture that makes up 57 islands that are located at the most southern end of Japan closest to Taiwan.

Planning to retire in Japan

  1. You will enjoy living in Japan if you make sure that you live in a smallish town that doesn't have much traffic. Ensure that there are no industrial sources within 50 kilometres (approximately 30 miles) of your location.
  2. Make sure you move to a location that is near sea level since the acidic precipitation is not as dangerous as it is at higher elevations. Also make sure that this location isn't downwind of any industrial region.
  3. You can live on the western side of Japan, but remember air pollution is worst in this location during the winter.
  4. Most high elevation mountainous locations are greatly impacted by acidic fog and thus should never be considered as a location for permanent residency. There has been numerous atmospheric chemistry studies showing that many mountains situated throughout rural parts of Japan record severe acidic fog during the warmer months. This acidic fog (pH < 4) causes vegetation damage in areas located near the summit of these mountains.

Planning a hike into the mountains - beware the clouds

Map showing location of Mt. Norikura in comparison to Nagoya and Osaka

If you're planning to go bush walking up to the summit of the mountains mentioned below during the summer, then be sure you do so when the mountain is cloud free.

Numerous people have complained that they experienced a burning sensation on their skin and in their eyes as soon as they walked from a cloud free part of the mountain into mountain fog. This fog ( cloud droplets) is acidic and can cause major eye and respiratory problems.

Below is a list of some of the lowest recorded pH levels at a number of mountains in central Japan:

Side note: The lower the value of pH, the more acidic the precipitation is.

Map showing location of Mt. Norikura in comparison to Nagoya and Osaka

Mt. Fuji (pH ~ 4.0)

Mt. Norikura (pH ~ 3.30) - Smog from both Nagoya and Osaka is transported by the wind to Mt. Norikura

Mt. Rokko (pH ~ 2.30)

Mt. Oyama (pH ~ 1.95) - Approximately the same acidicity levels as the hydrochloric acid found in your stomach


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  8. Shima M, Adachi M, Tanaka T and Tsunetoshi Y. Serum complement levels in children in communities with different levels of air pollution in Japan. Archives of Environmental Health. 1999;54(4):264-270.

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