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