It’s no surprise to residents of the country’s capital that Delhi’s air quality can be terrible, but now they can know just how dangerous it is to go outside.

The air quality index from the Central Pollution Control Board is the first national index to explain in layman’s terms the effect that breathing the air in 10 cities could have on a person’s health.

It started producing readings on Monday.

In Shadipur in western Delhi, for instance, the air on Monday morning contained three to four times the recommended level of small airborne particles – pollutants linked to serious health problems, including lung cancer.

The air quality index translates detailed information on different pollutants into a color-coded warning system. Hours where pollution was below regulatory limits are colored green to represent minimal impact on health, the hours in shades of red indicate levels that would affect healthy people and have a serious impact on those with pre-existing conditions.

The index, overseen by the environment ministry, calculates the average pollution for a day, which the government says is a better way to judge air quality.

If a resident of western Delhi wanted to avoid breathing in the worst air on Monday, they would have had to stay indoors until around 8:30 a.m. when the air quality had become less harmful, according to the data.

Pollution levels in that area fell as the day wore on, helping to push up the average air quality to “moderate,” or yellow, on the Indian government scale. Moderately polluted air means that it would cause “breathing discomfort” to people with asthma as well as those with diseases of the heart and lungs.

Environmentalists welcomed the move but said it wasn’t enough just to publish the data.

“We like the idea that the government is now making the effort to inform people about health hazards on a daily basis, in a language people understand,” said Anumita Roychowdhury, executive director of the Centre for Science and Environment, a New Delhi-based organization that lobbies for stronger protection for the environment.

Ms. Roychowdhury hopes that the data will make people realize that “hard decisions must be taken to clean up the air.”

Air pollution in Indian cities can be terrible, but it’s also predictable.

“In December, 65% of the days were severe [on the government scale], with pollution levels eight to nine times regulatory limits,” Ms. Roychowdhury said. “In March, there were no severe days.”

Cold winter air means that the cities pollution stays where we can breathe it instead of rising into the stratosphere, and the damp fog helps capture all the dust from coal fires and car tailpipes. “There’s an obvious seasonality to it,” Ms. Roychowdhury said.

The next step is for the government to use this data to do something about the pollution. It could follow the lead of other countries but pollution controls might not go down well with Indian car owners—or those celebrating wedding or religious holidays.

In Beijing, the government restricts what days you can drive your car if there are too many consecutive days of high pollution. They shut down factories and prohibit the use of firecrackers and barbecues.

Paris chooses to make public transportation free on days with high pollution.

Indian cities need similar ways to react to pollution and bring the levels down, Ms. Roychowdhury said. “They need to be more proactive about disseminating information, and not just put it on a website,” she said.

via How India’s New Air Quality Index Works – India Real Time – WSJ.