The growth of mould is caused by high temperatures and humidity. Specifically, temperatures of 15℃ or higher and humidity levels of 75% and above -- conditions found in Japan especially during the long, hot summer months. In addition to humidity during the months from June through September, humidity levels are also high inside modern Japanese buildings during the winter months. These factors create conditions for mould to grow all year round, not just during the summer.

Allergies and Mould
Mould may not seem very harmful, but it actually can have negative affects on people. Mould can cause nasal inflammation (allergic rhinitis), conjunctivitis (pinkeye) and asthma among other medical conditions. It can particularly be a problem for children or people with low immune systems thus causing serious health issues. In worst case scenarios mould can affect the lungs and brain, so it should not be taken lightly.
Mould spores float in the air and then settle and grow. In areas where water is concentrated, kitchens and bathrooms, humidity is at its highest thus allowing for the growth and spread of mould. Toilets and bathrooms are particularly difficult areas to keep free of mould as air flow in modern Japanese buildings is poor. So if you find mould, it is best to clean it up right away to prevent further growth.

How to Get Rid of Mould
Keeping doors and windows open and the fan running all the time are highly recommended. Beyond that, using strong disinfectants like Ethanol, “domesuto” and “kabi-killer” are the only other reasonable options. (It is said that Ethanol is very good in preventing mould.) Using disinfectants regularly will help keep mould to a minimum, however once a month or so one should spray strong disinfectants in water concentrated areas and leave it for a while before rinsing away. Please remember to use caution when cleaning with disinfectants as they give off strong fumes. It goes without saying that children and pets should not have contact with such disinfectants.
Fans in Japanese homes and apartments are not very strong, so it is a good idea to put out a separate fan when you are using strong cleaners. Wearing a mask, appropriate clothing (the cleaners can bleach out clothes), long sleeves and gloves are recommended, will help protect you. Lastly, be especially careful NOT to mix different cleaners when refilling bottles or cleaning as chemicals can react and cause dangerous fumes.

Futons, Closets and Mould
Did you know that humans lose about 1 liter of water a day? During the day and while sleeping our clothing and beds/ futons collect dead skin and absorb the moisture we lose by sweating. In order to keep clothing and bedding clean, it is important to air out such items out regularly - particularly futons, blankets and pillows. Hanging futons and bedding out in the sun is a very effective way to get rid of built up moisture, dust and to prevent mould.

Futons On sunny days people hang futons out on their balconies and beat them with an item called futon-tataki (futon-duster) to get rid of dust. For those people who really want to clean their futons, sending them off to the cleaners or washing them at a coin-laundry shop are other options. In the case you wash futons and blankets on your own, be sure to run them through the dryer thoroughly and then hang them outside in the sun (preferably on a hot, sunny day) for some time. As humidity levels are highest in the morning and after 3pm, it is best to hang out futons, bedding and clothing from about 10am to 3pm. On rainy days and the day after, humidity levels are high so leave your futon out (on the floor) for about an hour before putting it away, this will help it dry out a bit.

Closets Airing out closets is very important too as it does little good to hang out one’s bedding and then store it in a humid environment. Regularly opening closet doors and airing them out or running a fan in front of closets are good ways to deal with humidity. Cleaning out closets and running a vacuum cleaner also help to cut back on dust and mould.

Humidity Countermeasures
Products that absorb moisture are all the rage and there are various items available at stores and online.

Newspapers: If you are the type of person who likes to use items that are already on hand, then here is a simple method - newspaper. Roll up fairly recent newspaper (old papers have absorbed moisture and aren't as effective), rubber band them and then place them standing up in closets. Every couple of months be sure to replace the newspaper and you will have a very reasonable and handy way of dealing with humidity without having to buy special items.

Wooden Racks: For those people with futons, placing wooden racks (sunoko = すのこ) under futons and bedding in closets will help keep moisture away and keep mould from growing. Slipping sheets of newspaper underneath the racks, and changing them every couple of months, will also help prevent moisture build-up. Such wooden racks can be purchased at home centers for under 1000 Yen.

Absorption Goods: If you would like to try some of the items available out on the market, one particular company puts out items under the name "Dry Well" (ドライウェル) in the form of sheets and pads that absorb moisture. Putting these items in one's closet, dresser, or storage box can help deal with humidity. The reusable blue colored pads turn pink after they are full of moisture thus letting people know that it is time to hang them out in the sun for 2 or 3 hours. Afterwards the pads can be used until they turn pink again. These items may be purchased online starting for around 6000 Yen.