A Dragon in my Garage. The myth of the 9 m2 of green space per capita

In a classic book called The Haunted-Demon world: science as a candle in the dark, Carl Sagan presents an appealing history of a person who claims to have a dragon in a garage. The problem with that Dragon is that it doesn’t give any material clue about its existence. Regardless of the insistence of Sagan on the many possible ways to test whether the Dragon is there physically, the “owner” always finds excuses to avoid such trials. The tale ends up concluding that there is no difference between a dragon that cannot be tested and not any dragon at all.

There is indeed an absolute relevance of empirical evidence in science. A scientific fact must be proven as such, in material and always reproducible manner. But the scientific compromise with evidence doesn’t stop there. It is actually also connected with another fundamental component of science: consolidated knowledge. That means what others have said about a particular phenomenon. I’m avoiding here the use of the worth “truth” because there is not such a thing as “the truth”, but we can use instead another famous quotation: standing on the shoulders of giants. A proper and sound scientific work must, necessarily, stand on previous work and, again, in that empirical evidence.

Unfortunately, that is not always the case, and there are many examples in the history of science, even some tasty ones that my students have enjoyed in my lectures. But there are also current examples, where unstained facts and myths circulate from paper to paper and are being wrongly reproduced but not tested, nor doubled checked with the original source.

Among the several urban myths, there is one particularly resilient in scientific writing: the 9 m2 of green space per capita. This is a statement that has been attributed to the World Health Organisation (WHO) uncountable times in scientific papers. Here are some notable examples.

“The WHO recommends a minimum amount of 9m2 of green open space per person (WHO, 2009)”


The statement refers to this report: “URBAN PLANNING AND HUMAN HEALTH IN THE EUROPEAN CITY Report to the World Health Organisation 2009 International Society of City and Regional Planners (ISOCARP)”.

This can be traced to this other one: “Enhancing Resilience of Urban Ecosystems through Green Infrastructure (EnRoute)” https://publications.jrc.ec.europa.eu/repository/handle/JRC115375, which states: “European citizens have access to about 18 m2 public green space within the boundary of their city; this is double the amount suggested by the World Health Organisation of 9 m2 (World Health Organization, 2010)” (pp 43). The WHO reference is this one:

WHO (2010) Urban planning, environment and health: from evidence to policy action – Meeting report -. World Heal Organ Reg Off Eur: 119. doi: http://dx.doi.org/10.1016/j.biomaterials.2008.12.054

That can be found here:


I have checked the cited sources and couldn’t find any mention of a per capita green space amount in any document from the WHO. If you are interested in the topic, I invite you to do the same exercise and prove me wrong.

Here more (non exhaustive) examples of scientific articles making the same mistake:

Most likely and unfortunately the authors of those scientific articles and reports didn’t check the original source themselves and therefore opted for the shortcut of just taking it from another published piece. Why bother? Seems more important just to rush and publish or perish.

Urban research is full of myths. That is a statement that I’m regularly doing in my lectures. I even use a nice picture of the loch ness to illustrate my point. The situation is worrisome because it reflects a lack of rigour in the acknowledgement of scientific contributions, and a sort of superficiality in the manner ideas are treated, cited and reproduced. Eventually, evidencing the laziness to read the scientific work prepared by our peers.

Urban research is full of myths…

This year I found again this false 9 m2 parameter in a PhD thesis that I was invited to evaluate. In my report I commented that this statement was falsa, suggesting to the student to remove it from the text. To my big surprise, the student decided not to delete the paragraph and leave it as such in the final thesis version. Shame on her, and on the excuses given during the thesis disputation. Moments like that are those when I feel deeply sad for the sake of science, and I must console myself remembering that in the end, we are just simple humans prone to fail. But I keep the hope, that new generations, fresher researchers will keep a high scepticism, even if it has been published in the most respectable journal or has been said by a very famous scientist or referred to in an EU webpage. Science is a collective endeavour. The only way to build up new scientific knowledge is to stand over the shoulders of giants, but we must verify if those giants have said what many others claim they did.

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