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Is Summer Air Better than Winter Air?


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


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 Pollution & Effects on Fertility and Newborn Health

Last month, Dr. Shruti Mahalingaiah from the Boston University School of Medicine, along with other scholars published a study that examined the effects of exposure to air pollution on infertility. Researchers studied a sample of over 36,000 female nurses in the United States between 1989 and 2003. They found that women who lived within 199 meters of a major road were more likely to experience infertility. The study measured both primary and secondary infertility, where the former refers to women trying to conceive for at least a year without success and the latter refers to couples who cannot conceive after at least one prior pregnancy. Though there was no significant impact of air pollution on primary infertility, women who lived near major roads were 21% more likely to report secondary infertility compared to those who lived farther away. The infertility effects were stronger for those who experienced chronic exposure to air pollution rather than short-term exposure. 

Dr. Mahalingaiah indicated that the risks are small at an individual level. However, Mark Nieuwenhuijsen, a researcher at the Center for Research in Environmental Epidemiology and the Barcelona Institute for Global Health, told Reuters the finding could have a societal impact if a large number of women are exposed to air pollution. Given that a large number of major Indian cities, especially north Indian cities, are among the most polluted in the world, there could be large-scale infertility impacts seen here. However, the U.S. study is one of the first of its kind for tracking air pollution and infertility outcomes over a long-term period, and similar research is yet to be done in India.

Though infertility research is limited here, some work has been done on the impact of air pollution on the health of newborns. In 2014, scholars associated birth delivery data with nearby air pollution quality in Delhi for over 10,500 birth records. The study suggested that dangerous gases commonly found in polluted air could affect the health of newly born babies, leading to weight complications or premature death. Though more robust studies are required to show causation rather than correlation, another previous study by Fleischer et al. conducted across 22 countries suggests that air pollution may lead to low birth weights among babies, but not necessarily premature birth unless air pollution levels are extremely high. The bad news is that air pollution levels are, in fact, extremely high in most Indian cities, which could increase the number of preterm births; India already has the highest number of premature births globally.  Other studies conducted outside of India confirm findings on low birth weight and in some cases, high infant or perinatal mortality.

Overall, there is a wealth of evidence (much of which we haven’t listed here) which suggests negative effects of air pollution on fertility and perinatal or infant health. Though many of these studies need to be replicated in India, it’s likely that the effects will be no different here. In order to reduce negative health outcomes of air pollution, doctors suggest limiting outdoor activity and physical activity when possible. For cleaner air indoors, try one of our air filters!