March 05, 2019

Does a "green" environment prevent mental illness?

Articles claiming all sorts of health benefits from living in a leafy environment pop up from time to time. The latest one is below. And I don't doubt that a green environment can have a quite strong soothing effect on some people -- perhaps to the point of alleviating neurotic symptoms. But what is the general effect and how strong is it?

For a start, the effects in the study below were very weak, with odds ratios all just above 1.00.  Ratios that low are conventionally held to be incapable of supporting causative inferences.  Such ratios in the epidemiological literature are often greeted with war whoops and general jubilation but the fact of the matter is that they are verging on non-existence and are unlikely to replicate. Non-replication is in fact the bane of medical research so the chance of a marginal effect replicating is vanishingly small. So I could terminate my critique right there and  say that there was nothing of interest going on in the study concerned.

But what are we to make of claims such as the risk of illness being "55% higher" for people with little greenery around them.  It sounds impressive, does it not?  An obviously strong effect?  However you just have to ask "higher than what?" to see that we are being hornswoggled. If the effect is super weak to start with, a 55% advance on it is not much is it?  55% more than tiny is still tiny. Those percentage claims are the strongest and most impressive claims in the article but are totally deceptive.  We are being scammed.  It would not be too  strong to call the claims "slimy".

I don't know if I should go on but there are other lessons in the article about what not to do.  And another fault in the research is something very commmon among epidemiologists:  A total lack of curiosity.  They take a bit of data and use it without asking how that bit of data arose.  In this case they look at the amount of green space kids had around them without asking WHY some kids had more or less green space than others.  And that can lead to a total misunderstanding of what was going on in the data.

So why would some kids be growing up in leafier areas?  The obvious explanation is $$$$ -- money.  Leafier areas tend to be more prestious and hence more expensive to live in.  Poorer people live beside the tracks.  And we know that richer people are healthier.  They tend to live years longer than poor people, for instance.  The authors below did control for socio-economic status but income is not normally included in status indices and the two are not substitutable for one-another.  See Table 4  here

But income is only one explanation.  A factor with strong health correlates that is almost never examined is IQ. Smarter people might be better at or more interested in moving to a leafier area.  So it is the better health of high IQ people that was being observed in the study.  The effects in the study were so weak that we could have been observing nothing but the better health of high IQ people.

So good try but no cigar

Residential green space in childhood is associated with lower risk of psychiatric disorders from adolescence into adulthood

Kristine Engemann et al.

Abstract

Urban residence is associated with a higher risk of some psychiatric disorders, but the underlying drivers remain unknown. There is increasing evidence that the level of exposure to natural environments impacts mental health, but few large-scale epidemiological studies have assessed the general existence and importance of such associations. Here, we investigate the prospective association between green space and mental health in the Danish population. Green space presence was assessed at the individual level using high-resolution satellite data to calculate the normalized difference vegetation index within a 210 × 210 m square around each person’s place of residence (∼1 million people) from birth to the age of 10. We show that high levels of green space presence during childhood are associated with lower risk of a wide spectrum of psychiatric disorders later in life. Risk for subsequent mental illness for those who lived with the lowest level of green space during childhood was up to 55% higher across various disorders compared with those who lived with the highest level of green space. The association remained even after adjusting for urbanization, socioeconomic factors, parental history of mental illness, and parental age. Stronger association of cumulative green space presence during childhood compared with single-year green space presence suggests that presence throughout childhood is important. Our results show that green space during childhood is associated with better mental health, supporting efforts to better integrate natural environments into urban planning and childhood life.

SOURCE





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