Urban greening: what are the health risks for local populations?

Faced with the impact of cli­mate change, urb­an green­ing is one of the adapt­a­tion solu­tions being pro­moted. By increas­ing the num­ber of nat­ur­al ele­ments in cit­ies, such as veget­a­tion and water bod­ies, this approach helps to meet sev­er­al of the Sus­tain­able Devel­op­ment Goals (SDGs) adop­ted by the United Nations in 2015. Reve­get­a­tion improves the well-being and health of city dwell­ers and reduces nat­ur­al risks such as flooding.

In its latest report¹, the Inter­gov­ern­ment­al Pan­el on Cli­mate Change (IPCC) high­lights the major con­tri­bu­tion it can make to redu­cing extreme heat in private and pub­lic spaces: thanks to the shade provided by trees, elec­tri­city con­sump­tion linked to res­id­en­tial air con­di­tion­ing can be reduced by more than 30% at peak full stops. In Par­is, a study shows that the air tem­per­at­ure in the streets can be reduced by more than 4°C if three adapt­a­tion meas­ures are deployed: cre­at­ing parks, insu­lat­ing build­ings, and using reflect­ive mater­i­als². Today, more than half the world’s pop­u­la­tion lives in cit­ies. By 2050, this could have ris­en to two-thirds of the world’s pop­u­la­tion³.

Urb­an­isa­tion erodes biod­iversity. In the city, only the spe­cies that are most tol­er­ant of urb­an­isa­tion are present, and they are fairly homo­gen­eous from one city to anoth­er. Bring­ing nature back into the city cre­ates new eco­lo­gic­al hab­it­ats that bene­fit oth­er spe­cies. Plant­ing trees, adding green facades or roofs, cre­at­ing green spaces; there is a whole range of solu­tions for urb­an green­ing, with vary­ing degrees of impact on biod­iversity. Pol­lin­at­ing insects such as bees and but­ter­flies bene­fit from the pres­ence of urb­an park ⁴ and nec­tar-pro­du­cing plants⁵. The cre­ation of open mead­ows increases the rich­ness of bird spe­cies⁶. Wild mam­mals such as wild boar can also pen­et­rate cit­ies when green cor­ridors are created.

The changes to urb­an biod­iversity brought about by urb­an green­ing can increase the risk of the cir­cu­la­tion of vec­tor-borne dis­eases [editor’s note: infec­tious dis­eases trans­mit­ted by insects and mites, such as mal­aria, dengue fever, etc.]. Between 2009 and 2012, Mad­rid saw a resur­gence in cases of leish­mani­as­is. This was linked to an increase in the pop­u­la­tions of hares and flies, com­bined with great­er use by res­id­ents of a recently developed urb­an forest on the out­skirts of the city. In 2014, a dengue fever epi­dem­ic broke out in Tokyo: the link was estab­lished with vis­its to Yoyogi Park, where the tiger mos­quito was very present thanks to the hab­it­at provided by the park. But these health effects are highly loc­a­tion depend­ent. In Brazil, dengue is car­ried by anoth­er spe­cies of mos­quito (Aedes aegypti), which finds the con­di­tions it needs to thrive even in highly min­er­al­ised envir­on­ments. Unlike in Tokyo, research in São Paulo has shown a reduc­tion in dengue con­tam­in­a­tion in the city’s cool­er, veget­ated neigh­bour­hoods⁸.

Mam­mals encour­age the estab­lish­ment of ticks in veget­ated urb­an areas, which are them­selves vec­tors of patho­gens asso­ci­ated with dis­eases such as tick-borne enceph­al­it­is, haem­or­rhagic fevers (such as Crimean-Congo haem­or­rhagic fever) and Lyme dis­ease. There has been an increase in the incid­ence of some of these dis­eases in urb­an areas on Staten Island (New York) and in Europe. A study in the journ­al Infec­tion Eco­logy & Epi­demi­ology⁹ reports sev­er­al obser­va­tions: a large num­ber of ticks (136 every 100 metres) have been found in parks in Hel­sinki, with an infec­tion rate of between 19% and 55% with the bac­teri­um respons­ible for Lyme dis­ease. In Bav­aria (Ger­many), ticks car­ry­ing the para­site respons­ible for babesi­o­s­is are found in parks with per­man­ent deer populations.

Yes, this is one of the factors in the spread of dis­ease. We’re see­ing an increase in the use of veget­ated areas in towns and cit­ies. Increased human con­tact with biod­iversity encour­ages a resur­gence of vec­tor-borne dis­eases. This com­plic­ates the man­age­ment of green spaces by the pub­lic author­it­ies and hampers pub­lic accept­ance of urb­an green spaces. The Lyon met­ro­pol­it­an area is heav­ily inves­ted in this issue. Lawns in urb­an parks are no longer cut short to pro­mote biod­iversity. The city has there­fore installed signs to make walk­ers aware of the risks asso­ci­ated with ticks. They are reg­u­larly moved to main­tain a high level of pub­lic awareness.

The impact of green­ery on res­pir­at­ory dis­eases has been extens­ively stud­ied. Urb­an green­ing reduces green­house gases, pol­lu­tion, and heat. What’s more, parks encour­age phys­ic­al activ­ity: all of which reduces car­di­ovas­cu­lar risks. Con­versely, green­ery exposes people to pol­len and increases the risk of res­pir­at­ory aller­gies. But the risks asso­ci­ated with vec­tor-borne dis­eases are less well stud­ied. Urb­an eco­logy is a sci­entif­ic dis­cip­line in which a great deal of work has been done. How­ever, our know­ledge is patchy: we still do not fully under­stand the impact of urb­an green­ing on spe­cif­ic diversity (the num­ber of spe­cies in an envir­on­ment) and func­tion­al diversity (the vari­ety of spe­cies’ responses to change) in cities.

No, that’s not the aim of our research. Urb­an green­ing brings bene­fits to people, par­tic­u­larly in view of the increased risk of heat­waves because of cli­mate change. It is cru­cial to bet­ter under­stand the effects of urb­an green­ing to avoid related health risks. For example, what pur­pose does veget­a­tion serve for mos­qui­toes, and does it play a role in their rest­ing place? We don’t have the answer. What we do know is the value of increas­ing biod­iversity in cit­ies. For example, the dengue vir­us is car­ried by the tiger mos­quito, but cer­tain oth­er spe­cies of mos­quito do not spread it. When sev­er­al spe­cies are present, the risk of infec­tion is reduced.

This is a research and urb­an plan­ning ques­tion that we need to answer. Clearly, the water­ing of green spaces plays a major role: drip sys­tems pre­vent the form­a­tion of small col­lec­tions of water that are ver­it­able breed­ing grounds for mos­qui­toes. We are also work­ing on a research pro­ject in the sub­urbs of Mont­pel­li­er aimed at devel­op­ing an effect­ive bio­lo­gic­al con­trol strategy against the tiger mos­quito. We are study­ing the most appro­pri­ate sites (those that favour mat­ing) for releas­ing sterile male mos­qui­toes, depend­ing on the veget­a­tion. Oth­er aven­ues could also be explored to identi­fy the pred­at­ory spe­cies of the tiger mos­quito from the fae­ces of birds and bats, to be able to encour­age their pres­ence with a view to bio­lo­gic­al control.

Anaïs Marechal