Greener cities can help protect people and aquatic ecosystems from toxic chemicals

Our new paper showed that urban vegetation can "eat" some organic chemicals, supporting policies aimed at reducing emissions to help keep our world free of hazardous chemicals which are pumped in prodigious quantities into the air from our building insulation, computers, and couches.
Published in Earth & Environment
Greener cities can help protect people and aquatic ecosystems from toxic chemicals

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Our study, recently published in the journal Nature Communications, closely examines the movement of Organophosphate Esters (OPEs) through 19 major cities around the world. OPEs are chemicals that are used as plasticizers and flame retardants in many different products including cellphones, couches, and the insulation in homes and other buildings. Although not formally blacklisted by the Stockholm Convention on Persistent Organic Pollutants (POPs) — the main international agreement governing the use of the hazardous chemicals known as POPs — some OPEs are persistent in the environment and toxic to humans or to other organisms, and may be similarly hazardous as listed POPs. Studies have found that some OPEs can be carcinogenic, while others have been shown to decrease fertility, or to increase the risk of childhood asthma.

At the heart of our work is the insight that because different OPEs act like different kinds of POPs, we can use them to help us understand more broadly how OPEs and other POP-like chemicals move through our cities. Here we present the results of our work in tracking the sources of OPE emissions and their eventual paths through cities. To do this, we used the Multimedia Urban Model (MUM), which was developed by members of our team. We also offer suggestions for ways that we could reduce the harms caused by OPEs and other similar chemicals to people and to the world we live in.

We now pump out more OPEs than the compounds they replaced

If OPEs are known to be hazards, why did we start using them in the first place? In the mid-2000’s, OPE use greatly increased after another class of flame-retardant compounds were phased-out after being listed on the Stockholm Convention on POPs. And now we’ve gone overboard. Our first finding was that the 19 cities in our study emitted OPEs to the air at levels that likely dwarfed emissions of the compounds they replaced.

Traditionally, POPs are considered dangerous because they are Persistent (P), Bioaccumulative (B), and Toxic (T) to humans and ecosystems. But since some OPEs don’t fit this PBT framework, they are more difficult to regulate. Specifically, some OPEs are less bioaccumulative than the compounds they replaced and are part of the emerging class of Persistent, Mobile, and Toxic (PMT) compounds. PMT compounds are “mobile” in water; they are easily released to the environment via stormwater or wastewater. This means that people and ecosystems are exposed to OPEs and other mobile contaminants via water, and some OPEs have been found at relatively high levels in surface waters around the world, in groundwater, and, at lower levels, in treated drinking water, meaning that there is a significant potential for human and aquatic organism exposure.

Cities make chemicals end up in water, but this can be prevented with urban vegetation

Generally, we expect chemicals to act like they are supposed to – PBT chemicals should stay out of the water, and PMT chemicals should mostly stay out of the air. So we were really surprised when our results showed that the OPEs were behaving really differently in different cities. For example, the amount that was washed downstream through the Nile, Magdalena and Hooghly Rivers varied from only 4.3% in Cairo to 44% in Kolkata; while in Bogotá 52% of the OPEs were deposited on vegetation, where 39% were degraded. At the same time, the proportion of the chemicals blown down-wind from a city (top-right arrow in each panel of Figure 1) varied from 63% in Cairo, to 42% in Kolkata, to only 3.8% in Bogotá.

Fate of OPEs in Cairo, Bogotá, and Kolkata
Figure 1: Diagrams showing OPE fate for Cairo, Bogotá, and Kolkata in 2018. Dashed lines represent transformation processes, solid lines transport processes. Emissions (kg yr-1) are shown entering the lower-air compartment and fate process values are given as the % of total emissions. Values shown on each figure may not sum to 100 as only larger processes are shown. The trees, grass tufts, clouds, and city skylines were generated with the assistance of DALL·E 2

After some investigation, we found that these differences were caused by both the climates of the cities and by the way each of the cities had been built. Cairo was typical of a “sparse” city in an arid climate— here, chemicals released to the air were generally blown out of the city, where they would eventually be deposited in communities and ecosystems down-wind of the city. Bogotá was typical of a “densely vegetated”, humid and rainy city. Here, rain washed the OPEs out of the air, where they were caught by the large amount of vegetation. The vegetation then acted to hold in place and degrade some of the OPEs, effectively “metabolizing” or “eating” them. In Kolkata, a humid, “densely urbanized” city, OPEs were also washed out of the air, but because the city is densely urbanized they deposited on the sticky “urban film” (think window grease) which forms on buildings and other hard surfaces, and then washed off into the Hooghly River. This urban film pathway is extremely effective at moving chemicals into rivers because urban planners and engineers are understandably concerned about flooding, and use ditches or storm drains to move water off of our streets as quickly as possible. Unfortunately, this also means that chemicals in the water are also transported to downstream communities and aquatic ecosystems.

Reducing Harm

When it comes to reducing the harm caused by OPEsand other PMT and PBT chemicals like them—we first and foremost suggest implementing regulations aimed at reducing the amount we use these chemicals to deal with the problem at the source. Our results also indicated that OPE emissions likely came from many smaller sources rather than from specific manufacturing sectors, making broader restrictions on their use in products the most promising way to reduce their emissions.

At the same time, where possible we can support these policies by building our cities with more greenspace, increasing their ability to “eat” the chemicals they emit without exporting them downwind or downstream. If we redirect our stormwater to these green spaces, they can act as Green Infrastructure, engineered systems that use vegetation to help treat stormwater, wastewater, or other urban hazards, cutting off that efficient urban film to water pathway. Together, these changes can help us build “Sustainable Cities and Communities” one of the United Nation’s Sustainable Development Goals for Agenda 2030, and allow us to lead more sustainable lives.

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