It is well known that aerosols contribute to poor air quality and can harm human health 1, but not all aerosols are created equal. A new study in Nature proposes that focusing on certain aerosols, those with higher oxidative potential, might improve regulations aimed at curtailing the adverse effects of particulate matter (ref. 2). 

What are aerosols, and how are they monitored?

The term ‘aerosol,’ meant to be short for ‘air-solution,’ refers to any small air-suspended particle. Aerosols can be either solids or liquid droplets, and range in size from the microscopic to those visible with the naked eye. Their origins can be natural (like sea salt and dust) and/or human (like a nasty plume of car exhaust) (ref. 3).

As aerosols are essentially a category of things rather than something singularly defined, they are immensely complex to study, model and control. The typical metric used to monitor aerosols is their overall concentration in a region; sensible enough as the more there are, the more of them people will inhale. Researchers create maps that illustrate mass concentration and policymakers then seek to alleviate the densest regions (ref. 4). 

A more nuanced approach is to discern and locate which aerosols present the highest risk to respiratory and cardiovascular health. In a study published late last year, Daellenbach et al. sought to identify these by deducing their oxidative potential (OP), a substance’s ability to generate harmful reactive oxidative species (ROS), and their main primary and secondary sources in Europe. 

Oxidative Potential

Reactive oxidative species are molecules containing oxygen atoms with unpaired outer electrons which makes them highly unstable. They seek to ‘complete’ their electron pairs by snatching another molecule’s electrons, and things like cell walls and DNA, with their plentiful carbon-hydrogen bonds, are easy targets. Cells have defense mechanisms in place for these oxidative ‘attacks,’ as ROS do occur naturally in human systems, but when in excess cells can begin to deteriorate under oxidative stress (ref. 5).

The chemicals that make up or cover the surface of aerosols can either directly or indirectly introduce ROS into the lungs when breathing polluted air, and from the lungs those may travel to other organs. This is what most makes aerosols so toxic.  

Mapping the most harmful aerosols 

Using three novel biochemical assays and an air quality model with direct measurement inputs from sites in Switzerland and Liechtenstein, Daellenbach et al. found that aerosols with high OPs were not always concentrated in areas where overall aerosol concentration was also high. 

High OP hotspots were often located in highly populated places, i.e. cities and suburbs, likely due to these being primarily anthropogenic particles: metal emissions from automobiles and soot from fossil fuel or firewood combustion.  

Other regions with high(er) levels of aerosols had mostly particles with low oxidative potential, for instance nitrate, sulfate, and ammonium salts that get blown about by the wind from agricultural areas, and encompassed less populated areas (Figure 1).  

high oxidative potential reactive species map

Figure 1. From “Sources of particulate-matter air pollution and its oxidative potential in Europe” Daellenbach et al. 2019

By focusing on the most damaging aerosols, rather than on the most abundant, policymakers may then also simultaneously protect the greatest number of people. 

The body of research in regards to whether OP or mass concentration is the better indicator of aerosol health risk is limited and requires further empirical study (ref. 4). The future guidance from this work will be crucial as air quality and other environmental challenges continue to amplify, necessitating solutions that are actually achievable with often-limited time and resources.  

This article was written by Debbie Sanchez. Cover photo by Kouji Tsuru on Unsplash.

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