The Link Between Polluted Air and Sadness

The Link Between Polluted Air and Sadness

Air pollution is disastrous for the health of our body and brain. On highly polluted days, hospital admissions are higher than normal, and we know that external pollutants can enter our brain, potentially disrupting its function. But could air pollution affect our emotions too?

We’ve all experienced the joy of a deep breath of fresh air in the countryside. Compared to a dense city, rural life generally offers cleaner air and more pleasant odors. If air feels fresher as it enters our lungs, then it’s natural to expect a burst of happiness when we breathe in, even if it lasts only a few seconds. But the effect could be more meaningful than a superficial moment of comfort. To test this, researchers recently analyzed how air quality within cities correlates with expressions of happiness.

The researchers focused on 144 Chinese cities. To understand happiness, they investigated social media activity from the Chinese equivalent of Twitter. By inferring emotional sentiment from social media posts, they could explore how emotions reacted to air pollution changes in real-time rather than asking people how they felt after the fact. Memories can be notoriously unreliable, so analyses that avoid asking people about their past feelings are generally sensible.

Another bonus to social media messages is that they provide a more authentic record of beliefs and moods than a questionnaire would. People naturally and spontaneously offer their thoughts on Twitter because it’s a real-world platform that they use daily. They don’t need to work through a tricky experimental questionnaire designed by imperfect experimenters.

Data on air quality came from the Ministry of Environmental Protection in China. For each day of interest in their research sample, the researchers focused on the concentration of airborne particulates with a diameter less than 2.5 µm. Smaller particulates are typically more dangerous for physical health because they gain easier access to sensitive parts of our body. For example, airborne magnetite nanoparticles are derived from combustion and are toxic to the brain. But with a diameter of less than 0.2 µm, they can also directly enter our brain through the olfactory nerve which connects the brain to smell receptors in our nose.

To measure emotional tone in social media content, the researchers used a computer program to analyze the semantics in over 200 million tweets. They wanted to avoid tweets that directly referenced air quality itself, because they were most interested in mood changes that people didn’t consciously connect to pollution. So they excluded 0.05% of the tweets they collected, all of which contained words that could be linked to air quality. By accumulating and analyzing tweets according to their geographical location, a computer algorithm gave each city in the sample a happiness score for each day.

After testing the relationship between daily happiness scores and pollution concentrations in each city, the researchers found an overall negative correlation: higher pollution was associated with lower happiness. And in the cities with the most populous urban areas — Shanghai, Beijing, and Guangzhou — the negative emotional effects of rising pollution were almost three times stronger than the national data, suggesting that city dwellers suffered most from local drops in air quality. Women’s happiness was also more sensitive to air pollution than men’s happiness, although the reason for this difference was unclear.

The negative impact of pollution on happiness was consistent enough that every step up in pollution quantity, for example from light to moderate pollution, led to a decrease in happiness score. The researchers also noticed sharp drops in happiness when people received government alerts about severe pollution. This information mattered to people, and they used it to adjust their behavior in an effort to minimize their exposure to pollutants.

To test the generalizability of their happiness calculations, the researchers also checked how happiness scores fluctuated in relation to other variables. For example, they predictably found happier tweets on weekends and holidays compared to weekdays, and also happier tweets on days with good news (e.g. economic progress) rather than bad news (e.g. a natural disaster).

Perhaps most interestingly, they analyzed how happiness changed with the weather. Previous research in the US suggests that happiness scores based on social media activity tend to drop as rainfall increases, and they also follow an inverse U-shaped curve in relation to temperature: as temperatures increase, happiness also increases, but only up to ~70°F, after which happiness begins to decline again with greater warming. The research from the Chinese communities suggested a similar effect, although happiness scores peaked at a slightly colder temperature of 63.5°F before beginning to decline again.

On average, based on the researchers’ calculations, the happiness cost of a 1.8°F rise in temperature (above 63.5°F) was equivalent to a 1 μg per cubic meter rise in air pollutants. If you’re happier staying indoors with air conditioning on a hot day, you may also be happier staying indoors when it’s heavily polluted outside.

You’d be right to wonder whether the relationship between air pollution and happiness is driven by some variable other than the concentration of airborne particulates. Maybe traffic jams — which increase air pollution and decrease happiness — are actually the primary cause of people’s misery? Or perhaps increased pollution comes from excessive factory workloads on a particular day, which is what really depresses people?

The researchers came up with an ingenious way to rule out these effects. For each city, they calculated happiness scores based on pollution coming from external sources rather than internal sources, by modeling how wind patterns would drag in airborne pollutants from other cities that don’t impact local economic or social activity. They replicated their original findings with this new data, supporting their claim that low air quality really does directly harm happiness.

When we pollute the air in our cities, we don’t just increase our chances of health problems in the future — we increase our misery today. People in the study above weren’t simply complaining about pollution, because the researchers excluded any explicit social media commentary about air quality. They were expressing their general daily mood, and that mood was sensitive to the pollutants they breathed in when stepping out of their homes.

The study is only correlational, so even though the researchers covered several bases in trying to decipher the direct causal impact of pollution on mood, we can’t exclude the possibility of secondary factors explaining the effects. However, evidence on the physical and mental health costs of pollution keeps accumulating, and it never seems to provide good news.

If you live in a major city, there’s only so much you can do to get away from the poison cloud. You can avoid the busiest streets, and you can take frequent trips to the countryside to get some fresh air. But if you care about your city life and your city job, you may essentially be stuck with this regular downside. We don’t need to panic, but we do need to continue the search for cleaner transport and technology.

It’s often surprising when discussions focus more on the longer-term costs of climate change than the immediate costs of polluted bodies. Both of these are critical problems to solve, but it’s much easier to wrap our minds around quality of life here and now, which encourages more urgency in looking for solutions. So perhaps our feelings of sadness on a smoggy morning will motivate us to get away from the fumes.

The Link Between Polluted Air and Sadness

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