On a chilly September morning in 2021 in San Francisco, a group of firefighters packed their gear into a bright red emergency vehicle. In the air is the strong sweet smell of pine resin from a rapidly growing forest fire. It is on the verge of becoming the largest single fire in California history. The so-called Dixie Fire is spreading over the mountains of the Sierra Nevada and will ultimately destroy more than 3900 square kilometers of forest before it can be fully brought under control at the end of October.
Before the firefighters can support the 1000 other helpers in the extinguishing work, they still have to stop: In a small classroom in the training building of the fire brigade in San Francisco Firefighters and lets them fill out the questionnaire.
The pollution expert compares the samples with those handed in by the firefighters on their return. Prunicki wants to find out how the body reacts to the smoke. For example, do inflammations or changes in immune and heart function occur?
The smoke from forest fires is responsible for tens to hundreds of thousands of premature deaths around the world every year. Prunicki is one of several hundred scientists worldwide who are systematically investigating the health effects of smoke exposure. She wants to explain why smoke is more harmful to humans than other forms of environmental pollution.
The researchers examine both short and long-term effects. In addition, they deal with the question of who is exposed to the pollutants most and how to protect people and their health from the swaths of smoke, which can extend over thousands of kilometers after a fire such as the Dixie Fire.
An inferno can start from a tiny spark.
Such questions have become more urgent, especially in recent years, as climate change has exacerbated droughts and heat waves worldwide. There are now twice as many risky areas where even a small spark can set fire to dry vegetation and trigger an inferno as there were a few years ago. As a result, the frequency, scale and intensity of forest fires are increasing all over the world. Obviously, that's why the smoking periods last longer and longer.
Forest fires have devastated the west of the United States in particular in the past ten years - but some other countries have also had to deal with the worst fire disasters for generations. In 2021, a larger area burned in Siberia than in the same year through all other fires in the world. And Australia still suffers from the consequences of the devastating bush fires of the years 2019 and 2020, which are colloquially referred to as "Black Summer". Thousands of houses were destroyed and at least 30 people and hundreds of millions of animals lost their lives.
"The last 5 years are not comparable to the 10 to 15 years before," says Sam Heft-Neal, an environmental economist at Stanford University who studies the health risks of wildfires. We are now dealing with a completely different fire regime.
Forest fires: toxic smoke and smoke particles
The smoke from wildfires contains dozens of different particles, including soot and chemical compounds such as carbon monoxide. However, air quality experts are focusing primarily on the smallest particles in the smoke, which have a diameter of only 2.5 micrometers or less – that's 40th the average thickness of a human hair.
The firefighters who examined Prunicki are confronted with large quantities of this so-called PM2.5 enemy dust. According to Francesca Dominici, biostatist at the Harvard T. H. Chan School of Public Health in Boston, Massachusetts, every small amount of inhaled fine dust is questionable because it penetrates into the finest cracks of the lungs and comes into the bloodstream from there .
The body reacts to smoke in the airways as if there were germs or infections there, says Fay Johnston, an environmental epidemiologist at the University of Tasmania in Hobart, Australia. This is accompanied by a whole series of physiological changes: the hormone cortisol and blood sugar levels rise, which in turn makes the heartbeat more irregular and increases the likelihood of blood clots. The lung mucosa becomes inflamed, making breathing difficult.
Prunicki hopes to understand these physiological changes by measuring certain molecules in the blood-so-called biomarkers, which provide information about the current function of the respiratory tract and the immune and cardiovascular system. At the firefighters who are exposed to a high smoke load due to work, they would like to find out whether an exposure to the biomarkers is noticeable. And if so: Are the changes permanent and resemble those of people who are otherwise exposed to a high smoke load?
Near a fire, PM2.5 exposure can sometimes be more than 15 times higher than the 24-hour limit of 35 micrograms per cubic meter set by the U.S. Environmental Protection Agency (EPA). But many firefighters would not wear respirators when fighting fires because they are a hindrance, says Prunicki.
"Insufficiently high," says Francesca Dominici the value. "At this point you only breathe in a toxic soup." But even the statistics, even lower values, can be a serious health risk, because under such conditions, people tend to take protective measures. As Prunicki and colleagues were able to show, even in more than 100 kilometers away from forest fires, the smoke exposition can be so high that the people living there show a number of biomarker changes that are on a malfunction of the immune and cardiovascular Systems indicate.
In addition, smoke can make people more susceptible to infectious diseases or worsen the symptoms of respiratory diseases, such as Covid-19 or the flu. Dominici and her colleagues estimate that elevated PM2.5 levels during the 2020 fire season in Washington, California and Oregon may have led to about 19,700 additional Covid-19 cases and 750 deaths.
According to the most recognized hypothesis, the negative influences on the immune system and inflammation by PM2.5 are noticeable here. Affected people become more susceptible to infections, which leads to an increase in cases. In addition, the symptoms of the disease worsen. According to Dominici, a more controversial assumption is that some respiratory viruses-including SARS-COV-2-adhere to the fine dust particles and thus penetrate into the lung tissue.
Especially when millions of people are affected by smoke at the same time as during the "Black Summer" in Australia, the health effects are noticeable. The team led by Fay Johnston reported that the smoke of the fires led to thousands of hospital stays and hundreds of deaths due to heart and respiratory problems. After all, more than 80 percent of the Australian population were exposed to increased smoke loads.
In general, the research data shows that on days with high smoke levels, the number of people admitted to hospital with cardiovascular and respiratory diseases as well as diabetes increases – especially among children and the elderly. Pregnant women also have a higher risk of developing gestational diabetes or high blood pressure or giving birth to an underweight child: Heft-Neal estimates that nearly 7,000 premature births in California between 2007 and 2012 can be attributed to exposure to wildfire smoke during pregnancy.
Socioeconomic status also plays a role here, says Dominici. Because people in poorer areas were more likely to work outdoors, for example in agriculture or construction. In addition, they would be more likely to have other diseases and less access to adequate health care or air filtration systems. Due to these factors, forest fires would become an even greater burden for low-income regions.
What effects does the smoke load have over the long term?
Studying the long-term health effects of smoke on humans has proven difficult. Longitudinal studies that track people's health status after exposure would have to span decades. In addition, they should distinguish between the effects of many different environmental exposures.
In 2008, the immunologist Lisa Miller from the University of California in Davis offered a good, albeit sad opportunity to observe long -term effects in a controlled population. She was currently carrying out a study of allergies and asthma on a group of rhesus monkeys (Macaca Mulatta), when bad fires searched for North California. 25 newborn macaks were exposed to a PM2.5 load for ten days that was above the US limit.
Since this incident, Miller has monitored the health of the monkeys and that of their descendants. Compared to macaques that were born the following year, the 2008 group had a lower immune response and a lower lung capacity. In a study that has not yet been published, the immunologist team examined the lungs of the animals and found that the associated tissue had hardened, which may have influenced breathing. Perhaps the monkeys compensate for the limited breathing function in this way.
Miller thinks that this data also provides information on the long -term toxicity of the smoke of forest fires for the respiratory tract and the immune function of humans. You can say that it is harmful, she says. Nevertheless, the results could not be transferred directly to humans. Most are not outdoors around the clock like the monkeys from the controlled experiment. In addition, people could react differently to smoke than macaque.
Lack of unrestricted control groups in research
To determine the actual extent of smoke exposure from forest fires for humans, researchers are applying machine learning techniques to meteorological models and high-quality satellite data. With it, you can predict how the smoke will move through the atmosphere. In addition, sensors that measure the air quality in houses provide the researchers with a flood of real-time data. This allows you to track the PM2.5 values more and more accurately.
But more precise data does not solve the following problem: "The scientists run out of the comparison groups," says Lisa Miller. Bad air quality is omnipresent today-more than 90 percent of children around the world breathe in air whose PM2.5 values are above the limit values of the World Health Organization. The number of non -exposed people or primates for long -term studies shrinks continuously. This makes research all the more urgently, Miller thinks.
The research area is now particularly concerned with understanding which of the chemical compounds in forest fire smoke pollute the respiratory tract more than other types of air pollution and how these substances interact with human cells and damage them. We have been burning wood in our fireplaces since the beginning of time, she says. Nowadays, however, not only biomass burns, but also artificially produced material at the same time.
Take refuge in confined spaces.
Another important question, according to Heft-Neal, is how much the smoke enters the houses. In general, health authorities recommend that at-risk groups stay indoors on days with poor air quality and close all doors and windows. But these recommendations are "very general and not differentiated," says Johnston. In Australia, according to her, such advice is not particularly helpful, as the apartments there are usually not very well insulated.
Preliminary examinations in California houses with room air sensors have shown that the air inside contained fewer than half of the outdoor dust concentration. Nevertheless, the PM2.5 concentration in forest fires also rose almost triple indoors and thus exceeded the limit set for the 24-hour PM2.5 load specified by the US environmental protection authority. However, the infiltration rates are probably different from buildings to buildings. Therefore, you have to find out which types of houses are best suited for protection against air pollution, the booklet is found.
Colleen Reid, a health geographer at the University of Colorado Boulder, measures air quality in schools. These are critical places, as children are generally more susceptible to smoke from wildfires than adults. Reid and her colleagues plan to determine the air inside and outside school buildings in Colorado and compare it to that in students' homes. Under current U.S. public health guidelines, it's up to individual school districts to decide whether to stay open during events that increase air pollution. In such cases, it really depends on whether the air quality in schools or at home is better, says Reid. But this data does not really exist.
Their research could assist local authorities in the question of whether shelters should be set up with purified air. Schools could provide students there with clean air during the day – as well as the entire community if there are no lessons, Reid explains.
In order to develop such recommendations for public health, more measurement stations for air quality indoors are needed at the community level. This applies equally to individual surveillance, says Prunicki, especially for those who, like the firefighters examined, exposed to a high load. Her research on cardiovascular and immunological biomarkers could form the basis for firefighters and other endangered population groups one day with a simple blood test to determine whether they have achieved a dangerous level of lifelong smoke stress. Then you could take additional precautions to protect yourself from the smoke. Prunicki draws a comparison to X -ray technicians who wear dosimeters on the body that monitor their radiation exposure. "Why can't we do that with smoke?" She asks.
Tackling the issue at its source
The environmental epidemiologist Fay Johnston also finds it important to research the harmful effect of the smoke and to assess the individual risk. However, it points out that this kind of research does not deal with the root of the problem: climate change.
One could research endlessly and then recommend anti-inflammatory agents or subsidized air filters. "But we won't make any progress until we finally tackle the fundamental problem," she says, referring to climate change. She finds the path she is currently taking terrible.
Climate models predict that the 2021 fires in California and Russia are just the beginning. For example, according to a 2018 climate change assessment, if greenhouse gas emissions continue to rise, the area burned annually in California will increase by 77 percent by the end of the century. After all, there are efforts on the part of research and politics to avert this nightmare scenario.
Current forecasts show an enormous burden on healthcare due to rising emissions, since millions of people with limited airways, cardiovascular and immune functions may have to struggle, especially within the high-risk groups. The additional health costs related to premature deaths and hospital admissions that go back to the "Black Summer" in Australia estimate, for example, at two billion Australian dollars (around 1.3 billion euros). This amount is about ten times higher than that of the past few years, says Johnston.
Meanwhile, the scientists continue to work on determining the risks of a smoking exposure. The emergency services willingly participate in research, but fear the results. When Lieutenant Ken Smith opens the door of his fire engine, he states: »We don't think about what is in the smoke. If we knew what we breathe in in our profession, we would not be able to practice it. "
© Springer Nature 599, pp. 550-552, 2021
