The much-awaited monsoon season has finally arrived, marking an end to a year of winter smog and the overbearing summer heat. The rains also provide some respite from polluted air in many Indian cities—or so it seems. Clear, smog-free skies, gusty winds and a grassy, fresh smell offer a brief spell of clean air.

However, air quality data over the last two years shows that monsoon air pollution levels are still significantly worse than those recommended by the World Health Organization (WHO). Although monsoon air is cleaner than air during other seasons, air-pollution levels are worse than what is considered safe and remain a risk to public health.

To understand seasonal air pollution, we examined air quality data published by the US embassy in New Delhi and its consulates in Mumbai, Chennai, Hyderabad and Kolkata. We broke down the data between June 2014 and June 2016 into four seasons: winter (December to February), summer (March to June), monsoon (July to September) and post-monsoon (October to November). We then calculated the average PM 2.5 levels for each season. PM 2.5 measures particulate matter that is less than 2.5 micrometres in diameter—particles so small they can penetrate deep into the respiratory system and pose significant health risks.

It is important to note that our data only covers areas where the US embassy has its monitors—Chanakyapuri in New Delhi, Bandra in Mumbai, Ellaiamman Colony in Chennai, Patigadda in Hyderabad and Park Street area in Kolkata. Since air pollution may be localized to particular areas, the data may not indicate trends in other areas of a city.

In all five cities, no season had an average PM 2.5 level that met the WHO’s 24-hour exposure guideline of 25 micrograms per cubic metre. Delhi ranked worst with a high of 206 µg/m³ of PM 2.5 during winter and a low of 68 µg/m³ during monsoon. Hyderabad ranked the “best" with a high of 82 µg/m³ of PM 2.5 in the post-monsoon season. The city had its lowest seasonal PM 2.5 in the monsoon at 46 µg/m³. Chennai had the “best" yearly average of PM 2.5, ranging from 31 µg/m³ during summer to 98 µg/m³ during winter. The averages for Mumbai and Kolkata fell in between the other cities. Overall, even at their lowest PM 2.5 levels, Mumbai had 36 µg/m³, and Hyderabad 39 µg/m³ during monsoon season, indicating that even the best season doesn’t have safe levels of air pollution.

Seasonal variation in air pollution stems from a variety of factors. For one, post-monsoon season and winter have relatively lower wind speeds and air tends to accumulate at lower heights without mixing, which causes pollution to linger. Moreover, these seasons see higher emissions, especially during festivals such as Diwali and New Year, when fireworks pollute the air further. Both winter and early summer air is also polluted by post-harvest burning, especially in Delhi, which receives particulate emissions from burning crop residue in Uttar Pradesh, Punjab and Haryana.

But it’s important not to lose sight of the bigger picture. Even the relatively “good" air quality during the monsoon poses significant health risks. It is only “clean" compared with the toxic winter and post-monsoon air, but not nearly clean enough to be considered healthy. Though summer and monsoon may not create immediate symptoms that are common during winter and post-monsoon air quality—namely worsened asthma, shortness of breath, sore throat, migraines or clogged lungs—long-term exposure to air containing PM 2.5 above 25 µg/m³ has much worse effects. These include asthma, stroke, damage to the nervous system, kidney damage, high blood pressure, lung and other types of cancer, and even lower cognitive functioning.

What can be done to protect oneself from air pollution? Apps such as those from Plume Labs or AQIcn.org can help people know how dangerous the air is around them. These apps can also give advice on the time of day when outdoor activities, especially exercising, may be suitable. Wearing an N95- or N99-certified mask (95% and 99% effective against particulate matter, respectively) whenever possible will reduce exposure, which is especially important when spending time outdoors in traffic or exercising. Air purifiers that use high-efficiency particulate air (HEPA) filters can provide protection indoors as long as rooms are well sealed from outdoor air. The outdoor air data suggests we should use these precautions year-long. Leaving protective measures to winter time, when the air is visibly polluted, may not be enough to reduce long-term health risks.

Yet, limiting exposure through masks and air purifiers is not enough. They come at a cost, leaving low-income families who may not be able to afford them exposed to toxic air. Policy changes are necessary.

Though small changes in policy, such as the odd-even scheme in Delhi, have at times reduced the burden of air pollution, systematic changes haven’t gone far enough. The odd-even scheme in Delhi, for instance, only measurably reduced air pollution during the January trial, but not the April trial. And the January reduction was relatively modest: researchers affiliated with the University of Chicago and Harvard University estimated that PM 2.5 concentrations were around 35 µg/m³ lower, on average, during the January trial. With an average January PM 2.5 of over 200 µg/m³, such reductions are nowhere close to bringing pollution levels within the WHO’s recommended 24-hour guideline of 25 µg/m³.

Moreover, odd-even schemes do little to deter car ownership, which is increasingly growing in cities such as Delhi. But even car-focused policies aren’t enough, as industrial pollution from brick kilns and coal-fired power stations also need to be better regulated. Though Delhi has made more efforts to curb air pollution than other cities, ad hoc and short-term policies enacted during winter time are unlikely to reduce the long-term health effects of poor air quality throughout the year.

While one should take measures to protect oneself in the short run, policy changes which generate substantial and sustained improvements in air quality are essential. Our health depends on it.

Jay Kannaiyan is the director of Smart Air India and founder of Jammin Global Adventures. Bhumi Purohit is an incoming PhD student of Political Science at the University of California, Berkeley, and research lead at Smart Air India.

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