Teenage depression linked to airborne ozone

The harmful effects of air pollution have been reported many times, and the stats speak for themselves: according to the World Health Organisation, breathing bad air kills 7 million people per year. But apart from its impact on physical health, something that's not been looked at in detail is how pollution may also affect mental health, especially in younger people who may be more vulnerable to its effects. This is a shortcoming that Erika Manczak from the University of Denver set out to explore. She explained to Chris Smith how she found a strong link between teenage exposure to ozone, which is one constituent of air pollution, and the development of depression…

Erika - The California environmental protection agency regularly collects information from air quality monitors on things like ozone. I was able to take that data and essentially map it onto a study we were conducting that was looking at adolescence in San Francisco over a four year period.

Chris - And the two overlap. You can then say 'Here's what the air is doing and here's what their mental health is doing.'

Erika - Exactly. Yes. We were able to take our participants' home addresses and identify which census tract they lived in and then take the state of California data and identify what are the average, ambient ozone levels for that census tract.

Chris - What relationship emerged when you did this?

Erika - We began following our adolescents right before the onset of puberty. When most of our participants were somewhere between 9 and 13 years old, we then followed those same teenagers over the next approximately 4 years, while each of them transitioned through puberty. One of the things that we found was that there actually was not an association between mental health symptoms and ozone at baseline when the adolescents were pre-pubertal. However, once they transitioned into puberty, we began seeing significant differences in mental health according to ozone level. Adolescents who are living in a census tract that had relatively higher levels of community ozone showed significant increases in depression over that 4 year period. Whereas adolescents who are living in communities that had relatively lower ozone exposure, actually didn't show any significant changes in their depressive symptoms.

Chris - Can you disentangle the effect of the environment? Because if I live in a really downhill place where my exposure to environmental pollution is likely to be high, equally my exposure to a raft of other factors; Crime, poor environment, poor outdoor space availability, and so on. Those could well be playing a role. Can you get apart those two different aspects to this?

Erika - Excellent question and something we were really concerned about trying to untangle, since this is essentially a correlational study. We can't firmly clarify whether our associations are causal, we can only look at whether these things go together. Within this study, we were able to also look at things like the number of families living below the poverty line, socioeconomic status of individual households, as well as things like how many people in a community had finished a high school education. We're able to show that our effects were not accounted for, by some of these other community level concerns.

Chris - This would argue then that there is genuinely something about exposure to this pollution, and whether it's the ozone or something else that's there, that you are not measuring, but it's there at the same time as ozone, either could be possible. But how do you think this might be happening then? What's the mechanism?

Erika - One of the things that we know from the research on physical health effects of ozone is that inhaling ozone can increase inflammation in our lungs, but also systemic inflammation throughout our body. In-turn, we know that systemic inflammation is associated with things such as asthma or cardiovascular disease. However, in psychology, we're also recognising that those same patterns of systemic inflammation are also linked to depression and depressive symptoms. Perhaps one of the biological pathways for our findings would be that inhaling ozone might increase inflammation in the body, which would then put adolescents at greater risk for depressive symptoms as they aged.

Chris - What's puberty got to do with it? You mentioned that there was not a difference at baseline. In other words, before they went into puberty, everyone sort of faring the same. Are you just more vulnerable when you go into puberty?

Erika - Yes. That seems to be the case at least for depressive symptoms. Thankfully levels of depressive symptoms in children prior to puberty are relatively low. However, once kids begin puberty, we do see a pretty significant increase in depressive symptoms, suggesting ultimately that there seems to be heightened risk during that pubertal change. Precise reasons for this heightened risk are a little bit unclear. It certainly might have something to do with hormones and also might have to do with really dramatically changing social environments. Within our study, our findings parallel this pattern pretty nicely, suggesting that prior to puberty we don't really see these risk associations, but once puberty occurs, we do begin to see some of these emerging.