The massive and deadly eruption of the Hunga Tonga–Hunga Haʻapai volcano stunned the world when it sent a huge plume of gas, ash and other debris hurtling into the atmosphere last weekend. The mushrooming brown cloud was clearly visible in satellite images.

As the plume drifted, ash fell on other islands in the Polynesian nation of Tonga, including the main island of Tongatapu. This has complicated the response to other damage caused by the eruption—particularly from the tsunami it generated—because falling ash covered the runway of Fuaʻamotu International Airport on Tongatapu. According to reports, volunteers have swept the runway so aid-carrying planes can land.

The ash also poses some health concerns, though not always in the ways people might think. To explain more about these concerns, Scientific American spoke with Claire Horwell, a professor of geohealth at Durham University and founder and director of the International Volcanic Health Hazard Network, an organization that provides evidence-based information and advice on the health hazards of volcanic eruptions.

[An edited transcript of the interview follows.]

Are there health concerns about people inhaling volcanic ash? How might that differ from situations such as breathing wildfire smoke?

When volcanic ash starts to fall, it can look a bit like snow, but actually it is made of tiny particles of rock. Some of the particles are sand-sized, but others are more like flour—it’s these small particles that can be breathed into the lungs.

People have an idea that these rock particles are glassy and sharp and may cut the lungs when you breathe them in. They are right that the ash contains volcanic glass (solidified magma), but at the tiny size of these particles, they are not sharp. They’re a bit angular, but they won’t lacerate you.

Ash is different from wildfire smoke because it is made from rock and not combusted vegetation. Wood smoke is more uniformly fine-grained than volcanic ash, so it tends to stay airborne rather than settling. It has been shown to be quite toxic, whereas both laboratory and clinical studies on volcanic ash have not shown it to be particularly harmful. Having said that, the World Health Organization (WHO) has said that all particulate matter (PM) can cause respiratory disease—and there’s evidence of PM affecting every organ in the body, so it is sensible to be cautious and to try to limit exposure.

Most healthy people can tolerate breathing even quite high concentrations of ash for a while. They might find it irritating—it may give you a sore throat and a cough, and it may give you itchy skin and sore eyes. We are more concerned about people with existing respiratory and cardiovascular diseases. They may find that their conditions worsen, and they should make sure they keep their medication with them, use it as prescribed and get medical support if they are experiencing troubling symptoms, such as wheezing and shortness of breath.

There is some concern that people could develop serious lung diseases from inhaling ash, but there is almost no medical evidence for this. This is mainly because people are not usually exposed to enough ash, or for long enough, to trigger the diseases. Luckily Tonga receives around 1.5 meters of rainfall a year, so we are hopeful that ash will be quickly washed away and exposure will be short, even if the volcano continues to erupt for a while.
Also, some minerals in ash may be more toxic than others. People are concerned about crystalline silica because it is a carcinogen and can cause a scarring lung disease called silicosis. But we have shown that there are good reasons why volcanic silica may not be able to trigger these diseases, and clinical studies—albeit a limited number—around the world have failed to find outbreaks of such diseases. The ash from Hunga Tonga–Hunga Haʻapai is not expected to contain much crystalline silica: the magma erupted from the volcano hasn’t got the right composition (it is “andesitic” or “basaltic andesitic,” so [there is] not enough silicon dioxide), and the eruption style also wasn’t conducive to its formation.

Usually in eruptions, the ash is a minor health concern compared to the far greater public health issues related to provision of clean water, sanitation and prevention of disease outbreaks in evacuation shelters.

How should people protect themselves from inhaling volcanic ash?

People should reduce their exposure to ash by staying indoors and closing doors and windows. This can be challenging in tropical climates such as in Tonga, where buildings may be designed to be highly ventilated. People can use plastic sheeting or bed sheets to cover large openings, but it’s important that the inside environment doesn’t get dangerously hot.

When people have to go outdoors—people will need to be cleaning up after the disaster, going to work, school, et cetera—they can lessen their exposure to ash by wearing effective face masks. They may also need to wear them indoors if the ash got inside the home, and they need to clean it up.

We conducted a study on the effectiveness of different kinds of face coverings and masks for protection from volcanic ash. We found that—just like protecting yourself from COVID-19, wood smoke and any other aerosol or particulate—the best kinds of masks were the certified respirators (N95, FFP2 or P2, depending on your location) because of the excellent filtration of the nonwoven material that they’re made from and their close fit to the face. Surgical masks performed pretty well, too, but they’re not designed to fit to your face, so they provide less protection overall. Cloth performed really poorly in our experiments.

If you can only get hold of a surgical mask, you can tie a piece of cloth over the top to hold it better to your face. And if you only have cloth, try folding it into several layers. That will improve its filtration a little. Some agencies recommend wetting masks, but we found no advantage to that, and sometimes it made the filtration worse.

Ash can settle into water sources. Are there concerns about water quality? And are there ways to address any contamination?

Yes. Some countries rely on catching and storing rain for their water supply or take water directly from streams and rivers or open reservoirs. These water sources can get contaminated with the ash. For the most part, the ash will make the water cloudy and discolored, which will make it unpalatable. Sometimes the ash can release potentially toxic elements—but for short durations, these are unlikely to be a health hazard.

In Tonga, outside of urban areas, some people are reliant on collecting their own rainwater. They should cover the tank but also make sure that ash isn’t being washed into the supply from their roof. They may need to disconnect the pipes and clean the roof and gutters before reconnecting to collect fresh rainfall. To supplement existing supplies, alternative sources of drinking water may need to be provided for a while.

The risks from chemical contamination of water supplies by ashfall are very much secondary to the considerations of water quantity (having sufficient water per person per day for drinking, cooking, washing dishes, handwashing and bathing) and microbiological safety of the drinking water. The risks posed by waterborne pathogens are more immediate and serious than any chemical risks. People should disinfect their drinking water by boiling, adding unscented bleach [as instructed by the Environmental Protection Agency or using filtration.

How might ash affect local agriculture and ecosystems?

There are initial reports of [several centimeters] of ash on Tonga. This ash will have settled on crops and other plants. The weight of the ash may have damaged them, and the ash layer will disrupt photosynthesis. The ash can also have an acidic coating, which can burn leaves. Hopefully rain will rapidly wash the ash off the crops. Ashy fruits and vegetables can still be eaten, but they should be washed first.

Livestock can also be affected by ash. Troughs or pasture may get covered in ash, so the animals may require supplemental feed and water. In general, though, livestock are at greater risk from starvation, dehydration and the physical effects of eating ash—including blocked intestines and rapid tooth wear—than from toxicological effects of the ash, although ash that is high in available fluoride is known to cause harmful fluorosis in animals if they ingest it along with their feed.

How long will the ash be a threat to public health?

That’s a hard question to answer. At the moment, we don’t know how long the Hunga Tonga–Hunga Haʻapai eruption will continue for. It’s possible that it could finish soon, but some eruptions can continue erupting for weeks, months or even years!

The fact that it rains frequently in Tonga and other Pacific islands will really help the nations that have received ashfall. Hopefully the rain will clear the ash from the cataclysmic eruption very rapidly and any future ashfalls. The rain will also help to suppress ash in the air, which can be remobilized by wind, transport or activities such as clearing the ash from roads and roofs.