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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.


How healthy are Delhi kids’ lungs?

From: The Hindu

In 2008, the Chittranjan National Cancer Institute (CNCI) in Kolkata published the results from a study examining the health of lung function among school children, aged 4-17, in Delhi. To test whether children in Delhi fared worse than those outside of the city, the study compared 11,628 children from different parts of Delhi to 4,536 children in rural West Bengal and rural Uttaranchal—notably less polluted areas—over the course of 3 years.

Children are thought to be particularly susceptible to air pollution. They have higher oxygen demands then adults, leading to higher respiration rates. Their airways are also smaller than those of adults, so they are more likely to get inflammation due to air pollution. And because their immunity is still building up, they are more prone to illnesses from pollution than adults. If they are exposed to chronic pollution, children may develop lung damage into adult life.

The results from the study, summarized below, indicate that Delhi children show significantly worse outcomes for respiratory health and lung function compared to those in the control group. Children from lower socioeconomic groups usually fare worse because they have higher exposure to outdoor and indoor pollutants. The study further stated that children spend 2/3 of their time indoors, meaning air quality at schools and homes have a significant impact on health.

Many of the solutions to improve health outcomes for Delhi’s children necessitate policy changes — ensuring that schools are built far away from polluting sources, stricter monitoring of polluting sources, and better access to health services, to name a few. But ensuring kids avoid physical activity during high-pollution hours and wear anti-pollution masks whenever possible can help. For clean air indoors, check out Smart Air Filters’ low-cost air purifiers.


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!


The Really Small Stuff

To test how effective the DIY filter is, I’ve been posting lots of data on 2.5 micron particles. But my particle counter also measures smaller 0.5 micron particles, and I’ve been looking closely at that lately.

Remember that the “PM 2.5” you hear in the news is different from the numbers on most particle counters. Laser particle counters count X particle size and above. Government machines estimate the weight (mass) of particles of X particle size and below. 

0.5 micron particles are tougher to get rid of, so how well does the DIY filter do? To answer that question, my collaborator Anna tested the DIY filter we’re shipping from Smart Air on three different nights in her home. Here’s the average amount of 0.5 micron particles the DIY filter removed, with 2.5 micron particles as a comparison:


As with my most recent test, I calculated percent reduction compared to the room air before turning on the filter, rather than compared to outside air. This is a more rigorous and conservative way to calculate effectiveness than comparing reductions versus outside air.

Conclusion: On average, the DIY filter got 92% of the 2.5 micron particles and 84% of the 0.5 micron particles. Although 0.5 micron particles are smaller and harder to catch, the DIY filter is still getting 84% of them.

(Update from the future! Want more data? I do! These results are replicated in 200 glorious days of a longevity test here.)

As always, I’m detailing my methods for data nerds here:


Methods: For the DIY tests, we used the same HEPA and Meidi fan we’re shipping from Smart Air. As in the tests I reported earlier, my Smart Air collaborator Anna took a baseline measurement of her bedroom air with the Dylos “beast” particle counter, then she turned on the DIY filter and switched the particle counter to take hourly measurements.

Anna took the baseline measurement before she went to bed, and she set the particle counter to take hourly measurements until she woke up in the morning. To calculate effectiveness, we averaged the 0.5 micron readings for the four hours prior to waking up, and calculated the percentage reduction from the baseline reading (before the DIY filter was turned on). As I’ve said before, I think this method of calculating effectiveness is more rigorous than calculating as reduction from outside because indoor air is cleaner than outdoor air.

As always, the DIY filter and particle counter were on opposite sides of the room. The filter was against the west wall. The particle counter was close to the east wall, on top of a sofa, like this:


Anna’s room is 15 meters squared. The windows were closed at all times, and the bedroom door was mostly closed before Anna went to bed, and it was always closed during sleeping hours.

The raw data is here:



If you look at the outside AQI, you’ll see that the first test (9/9) has the most stable outside AQI counts. Thus, that number is probably the most reliable. It also happens to be the average of the three numbers.


Filters and Fetuses

I don’t meant to be sensational, but I just got a picture from a friend of a friend who is pregnant and just started using the DIY filter three days ago. The filter is turning black already.

After all, scientific research suggests children are the most affected by air pollution. If the outrageous prices of filters are keeping pregnant women from cleaning their air, then the DIY filter has already started to change that.