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Is Summer Air Better than Winter Air?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

 

Summer is here, bringing with it clearer skies and certainly cleaner air. Right?

Summer always seems to drive out the dense clouds of pollution that suffocate many Indian cities. However, while summer air is in fact cleaner than air during other seasons, it’s still far from safe according to the standards set by the World Health Organization (WHO).

During the winter, cold air traps pollutants close to the ground, a process called an “inversion.” Summer heat prevents this inversion, which does improve the air quality. However, average air conditions in India are still clearly not ideal.

Here’s a map of today’s pollution levels across India:

 

pollution levels
Source: https://aqicn.org/map/india/

 

On a day like today, when the AQI in Chennai, Hyderabad, Kolkata, Mumbai and New Delhi is in the ‘unhealthy’ or ‘very unhealthy’ range, we often wonder at Smart Air if the pollution in summer really is any better than the winter.

We got to the bottom of it by analyzing the US Embassy’s data in New Delhi and US consulates’ data in Mumbai, Chennai, Hyderabad, and Kolkata. So is summer air really better than winter air? We took the data from the past two years (June 2014 to June 2016) and broke it down into four seasons: winter (December to February), summer (March to June), monsoon (July to September), and post-monsoon (October to November). Next, we calculated the average particulate pollution (PM2.5) levels for each season.

Across the five cities we looked at, PM 2.5 levels were 26% better in the summer—118 micrograms in the winter compared to 49 micrograms in the summer. That means summer air is better.

Let’s take a look at the difference in PM2.5 between the five cities during different seasons:

 

 

US Embassy Air Quality Data
U.S. Department of State Data, June 2014 – June 2016. Air quality data may not be validated or verified

 

But how good is “better?” Here in India, “better” is nowhere near “safe.” Over the course of the two years we analyzed, average annual pollution levels in all five cities never fell below even the WHO’s more lenient (24-hour) exposure limit (25 micrograms per cubic meter). In fact, the average pollution levels across all the cities we tested was about 500% the WHO annual limit (10 micrograms) and 200% of the more lenient 24-hour limit (25)!

 

The lowest summer pollution level we found was Chennai (31 micrograms). But even that lowest summer level still surpassed the WHO limits.

Below are the 2-year graphs for each city. You can see that each city has two distinct swells in PM2.5 levels during the winter, each followed by 2 clear dips during the summer. Interestingly enough, comparing the summer and winter levels of each city from 2014-2015 to 2015-2016 shows some cities’ PM2.5 levels improving, while others’ increase between years. Most notably, Chennai’s winter pollution levels dropped significantly between years as did Hyderabad’s, while New Delhi and Kolkata experienced clear increases. However, we’re not sure whether or not this improvement and worsening of PM2.5 levels can be attributed to cities’ environmental efforts (or lack thereof).

The conclusion? The evidence is quite clear: summer air is in fact better than winter air. However, despite all the blue skies and warm days we’ve been having lately, there’s still a need to protect yourself inside and outside the house. Don’t mistake “better” for “safe.” Neither summer nor winter air meets WHO health standards and summer air is still of significant concern to public health.

 

Chennai US Department of State
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.

 

US Embassy Air Quality
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.

 

US Embassy Air Quality Data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.

 

US Embassy Air Quality Data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.

 

US Embassy Air quality data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.

 

Air Quality Around the World

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

In the last couple of posts, we have mentioned that 13 of the world’s 20 most polluted cities are in India. The rest are in Pakistan, Qatar, Turkey, Iran, and Bangladesh. But the top 20 rankings made us wonder, how does India compare to the rest of the world? To find out, we selected a few cities from around the globe and mapped the World Health Organization’s 2014 air quality data. Due to variations in the frequency of data collection, WHO’s database contains data collected anywhere from 2010-2013, varying by city and country. The map below shows the outdoor air quality as measured by PM 2.5, or particulate matters that are two and a half microns or less in width. PM 2.5 are approximately 30 times smaller than human hair. Due to their size, they can lodge deeply into lungs and lead to health problems.

Air quality is drastically different across continents, but even cities with seemingly clean air see high-pollution days. Recently, the city of Stuttgart, Germany issued an air pollution warning and asked residents to leave cars at home in lieu of public transport to bring down pollution levels. Though Stuttgart averages healthy levels of air pollution on an annual basis, the city saw 64 days of pollution above EU’s recommended 40 µg/m³ for PM 10 in 2014. Stuttgart government issued the air pollution warning after pollution levels rose to 89 µg/m³ (PM 10) on January 19, 2016.

Similar to Stuttgart, other cities in Europe have taken measures to bring down air pollution. Even though these pollution levels are significantly better off than India’s, they are still above WHO and EU’s recommended levels. In 2014, Paris—like New Delhi—implemented an odd-even scheme to bring down pollution levels. Milan took more drastic measures in 2015 by banning all cars, motorcycles, and scooters between 10 am and 4 pm for a three-day period to bring down pollution levels. Contrastingly, London has introduced a congestion charge for those seeking to drive in the city during the day on weekdays. Beijing has also taken various measures, including putting limits on cars, factories, and construction sites on days with high levels of smog.

As Delhi government continues its debate on how to move forward with the odd-even policy, scholars from the Energy Policy Institute at the University of Chicago the Evidence for Policy Design group at Harvard University examined preliminary data to understand the pilot period’s impact. They found that the odd-even pilot reduced hourly particulate air pollution concentration by 10-13%, but doubted the scheme could work long-term. In Mexico City, a similar scheme led to worse pollution outcomes when households purchased second cars  or old, polluting cars to overcome the odd-even rule. The scholars called for a pilot on congestion charges akin to London’s to understand how this could lead to long-term reductions in air pollution.

As Delhi’s debates on how best to curb air pollution continue, however, many of the other cities that see dangerously high levels of pollution in India necessitate a louder public debate. Here is a chart showing the top 20 polluted cities around the world as compiled by the World Economic Forum. If you live in one of these cities, start the conversation!