Schematic of the interplay between vapor intrusion, the built environment, and phytoforensic processes.
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Can Trees Track the Air that You Breathe?

A new paper in PLOS One says trees outside your house can act as sentinels tracking the concentrations of potentially hazardous chemicals in your home’s air.

What is in the air that you breathe at home? It’s mostly nitrogen. There’s a significant amount of oxygen, along with some other chemicals. There are certainly some you’d want to track. For example, carbon monoxide is something you’d want to know about. That’s why you probably have a carbon monoxide detector. But what about other chemicals?

A new paper by Wilson and colleagues Phytoforensics: Trees as bioindicators of potential indoor exposure via vapor intrusion examines exposure to volatile organic compounds, known as VOCs, that are increasingly causing concern for environmental health.

The problem comes from organic compounds that leech into groundwater from industrial processes, like poor waste management. This pollution contaminates groundwater. The problem isn’t then about drinking water, but about the compounds evaporating from the water to become a health hazard as gas.

The usual method to see if a residence is at risk from VOCs is Sub-slab sampling, examining the soil beneath the foundations of a house to see if there is a build-up of VOCs that will then escape into the house and the air. The effectiveness of the sampling is going to vary by season and also by luck. The sub-slab environment can be very variable in VOCs, and the sample volume is relatively low. If the sample is taken from the wrong place, a danger could be missed.

Schematic of the interplay between vapor intrusion, the built environment, and phytoforensic processes.
Schematic of the interplay between vapor intrusion, the built environment, and phytoforensic processes. Image by Wilson et al. 2018

What Wilson and colleagues have examined is how trees react to VOCs. If they’re in a residential area, then their groundwater is comparable to the groundwater in sub-slab environments. Their sample volume, however, is defined by their root system. The roots mean that trees are less likely to be troubled by the luck of the draw regarding VOC hotspots. But do trees process VOCs in a way that can be tracked meaningfully?

Wilson and co-authors find that the answer is yes – but trees bring their own problems. You can place probes where you like – and that’s likely to be around the area that you think has a problem. It might seem a bit silly to say so, but you can only sample trees where the trees are growing. If your local administration has removed trees to tidy things up, then your record of VOCs in the soil has gone. However, where you can use them, trees seem staggeringly useful. Wilson et al report:

The rapid and non-invasive nature of tree sampling are notable advantages: even with less than 60 trees in the vicinity of the source area, roughly 12 hours of tree-core sampling with minimal equipment at the PCE Southeast Contamination Site was sufficient to delineate vapor intrusion potential in the study area and offered comparable delineation to traditional sub-slab sampling performed at 140 properties over a period of approximately 2 years.

This, they argue, means that a tree survey could identify where to expect problem areas and, if needed, you can target your probes more effectively. If probes are turning up odd results, the tree data would also suggest which probes might need re-siting.

It seems that the water trees drink can help track the quality of the air you breathe.

Alun Salt

Alun (he/him) is the Producer for Botany One. It's his job to keep the server running. He's not a botanist, but started running into them on a regular basis while working on writing modules for an Interdisciplinary Science course and, later, helping teach mathematics to Biologists. His degrees are in archaeology and ancient history.

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