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