Photo/Illutration Researchers measure carbon dioxide concentrations in a test using a train car of Tokyo Metro’s Fukutoshin Line. (Provided by National Institute of Advanced Industrial Science and Technology)

Opening windows on packed trains can reduce the carbon dioxide level by about 15 percent, according to a study that could be used to determine proper ventilation in buildings to reduce the risk of COVID-19.

The test was conducted on Oct. 21 and 22 by the National Institute of Advanced Industrial Science and Technology (AIST), based in Tsukuba, Ibaraki Prefecture, at the request of the transport ministry.

The researchers used an eight-car train of Tokyo Metro Co.’s Fukutoshin Line. They assumed the express train does not stop at all stations, and moves about nine minutes without opening or closing the doors.

They kept one of the train cars in the middle section empty, and released CO2 into the carriage through tubes. The level was equal to the amount exhaled if passengers had filled the car to 150 percent of capacity.

Twenty-one indicators in the car found the CO2 concentration rose to an average of 3,200 parts per million (ppm), or 0.32 percent, when the windows were closed.

If two windows were open about 10 centimeters, the CO2 concentration increased to an average of 2,700 ppm, or 0.27 percent, which was about 15 percent lower than the closed-window case.

The Japan Society for Occupational Health says an acceptable range of CO2 concentration that does not negatively affect workers’ health is 5,000 ppm, or 0.5 percent, or lower.

The law concerning building sanitation defines the standard for comfortable air quality as 1,000 ppm, or 0.1 percent, or lower.

As an anti-COVID-19 measure at bars and restaurants, the central government has said that it is effective to confirm if an indoor level of CO2 concentration does not exceed 1,000 ppm.

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