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Indigenous Amazon forests absorb noxious fumes and prevent diseases from wildfires, study suggests

2019 and 2020 saw massive fires in the Brazilian Amazon, largely experts say due to Bolsonaro's failed environmental policies. Image by Sergio Vale / Acre state.

  • A new decade-long study estimates forests in Indigenous lands in the Brazilian Amazon can potentially prevent about 15 million cases of respiratory and cardiovascular infections each year by absorbing thousands of tons of dangerous pollutants emitted by forest fires.
  • Forest fires are mainly caused by deforestation to clear the land, releasing noxious fumes which contain carbonaceous aerosol, the main component of fine particulate matter which enters the bloodstream and can cause heart disease and lung cancer.
  • Health impacts from forest fires are not only restricted to nearby populations. Intense smoke can travel hundreds of kilometers away from the point of origin.
  • The researchers say the study’s findings demonstrate the need for Brazil’s government to resume Indigenous territories’ demarcations and public policies.

SÃO PAULO—A new study published in Nature estimates that forests in Indigenous lands in Brazil’s Amazon have the potential to absorb over 7,000 tons of noxious fumes from forest fires every year, preventing about 15 million cases of respiratory and cardiovascular diseases annually, which would otherwise cost $2 billion to Brazil’s public health system.

The effect on the health of populations adds to the environmental impacts of fires in the Amazon forest, which are mainly caused by deforestation and contribute to increased emissions.

What was not yet known was the level of those damages, the costs and the ability of the Amazon forests in Indigenous lands to absorb the pollutants, said the study’s authors.

Fire-related incidence of respiratory and cardiovascular infections for the entire Brazilian Amazon on a municipality basis from 2010 to 2019. Spatially, the Arc of deforestation in the Amazon was the region with the highest average incidences of infections reported. Image courtesy of EcoHealth Alliance.
Fire-related incidence of respiratory and cardiovascular infections for the entire Brazilian Amazon on a municipality basis from 2010 to 2019. Spatially, the Arc of deforestation in the Amazon was the region with the highest average incidences of infections reported. Image courtesy of EcoHealth Alliance.

“The study is the first to measure how much the loss of rainforests protected by Indigenous peoples can cost human health,” said Paula Prist, lead author of the study and research scientist at the EcoHealth Alliance, during a press conference last week.

The study is part of compounding research pointing to the successful forest conservation efforts and environmental services Indigenous territories in the Amazon provide. The Amazon Indigenous lands ratified by the Brazilian state, which number 383, were included in the study.

Based on an analysis of ten years of data, from 2010 to 2019, the researchers found that the Brazilian Amazon rainforest could absorb 26,000 metric tons of particulate matter of small size (PM2.5 or particulate matter with a diameter smaller than 0.0025 mm) released by fires every year. Indigenous territories have the potential to absorb 27% of these pollutants, despite occupying 22% of the Amazon area, covering nine states within Brazil’s Amazon basin.

PM2.5 penetrates the pulmonary alveoli and can pass directly through the lungs into the blood system. They are associated with heart disease, stroke, emphysema, lung cancer, bronchitis, asthma, and chest pain, among other conditions.

The study approximates the absorption capacity of the Amazon forest areas and analyzes theoretical removal rates, which the researchers say may be overestimated due to weather factors. The actual smoke released by the fires in the rainforest is about 1.68 tons of PM2.5 annually.

For each hectare (2.4 acres) of burning forest, the expenses of treating associated illnesses are between $2 million and $8 million. These cases vary per municipality. During the study period, more than 1.4 million cases of respiratory and cardiovascular infections related to forest fires occurred in 772 municipalities of the Amazon, in addition to 168,663 cases in Indigenous territories.

The fires are caused mainly by illegal farmers, ranchers, and land grabbers clearing the land and torching trees to make way for agriculture or livestock.

“The evergreen broadleaf forests of the Amazon have among the highest emission factors for black and organic carbonaceous aerosol, the primary components of fine particulate matter,” said the study. “This forest characteristic leads to more severe fires that emit more carbonaceous aerosols.”

Read more: Survival of Indigenous communities at risk as Amazon fire season advances

Urgency in the forests and Indigenous lands

The authors of the study said the findings indicate that the protection provided by forests does not serve only Indigenous communities but populations in rural and urban areas. This often includes “the other side of the Amazon, [as in] the Arc of deforestation, [which] has lost the most forest cover to agroindustry and other legal and illegal activities.” The arc of deforestation is a region responsible for 75% of deforestation in the Amazon.

Using satellite data from two NASA fire systems, the researchers found that the smoke absorbed by the rainforest can reach up to 500 kilometers (310 miles) from the location of the initial fire.

A VIIRS image of the Amazon’s Xingu region on September 8th 2020, including the locations of VIIRS active fire detections. Shown are well-developed fire fronts for active understory fires in the Xingu and the buildup of smoke in the western Amazon. Image from NASA’s WorldView site.

Fire is not a natural element of the Amazon rainforest, as it is a humid environment even in the dry season from July to November, explained Marcia Macedo, associate scientist at the Woodwell Climate Research Center.

“Deforestation is a major source of ignition, and when the fire is very intense, the smoke rises high and can be transported to large population centers, as we saw in 2019 in São Paulo city. It was three o’clock in the afternoon [when the sky turned] dark due to fires in the Amazon.”

There was an increase in forest loss due to fires during the study period, a situation that has been getting worse. After the study, between 2020 and 2022, there were also record deforestation rates in the region. In 2022, 49% of the forest area burned in the country was in the Amazon, totaling 7.9 million hectares (19.5 million acres).

“The number of fires has been increasing in the last few years,” Prist said. “And in 2020, deforestation rates reached the highest levels of the decade in the Brazilian Amazon.”

Recent studies have shown that Indigenous-managed forests suffer less deforestation and represent some of the remaining carbon sinks in the Amazon Rainforest. These forests act as a biofilter for air pollution and improve air quality due to the leaves’ rough texture and large contact area absorbing pollutants.

Fire in Humaitá, Amazonas state, Brazil in August 2022. Photo © Christian Braga / Greenpeace
Fire in Humaitá, Amazonas state, Brazil in August 2022. Photo © Christian Braga / Greenpeace

“Everything we have been saying about the role of Indigenous peoples in combating global warming and climate change has been confirmed by scientific knowledge. The removal of tons of harmful agents from the atmosphere, benefiting the population at large, demonstrates the need to resume the demarcations and public policies of those territories,” said Dinaman Tuxã, executive coordinator of APIB, an umbrella group representing Brazil’s Indigenous people’s organizations.

Action against Amazon fires needs to start now, alerted Prist.

“At that time, the region becomes among the most polluted places on earth; the study shows that we have to do something before the next dry season starts.”

Climate scientist and co-chair of the Science Panel for the Amazon Carlos Nobre said the study must serve as an inspiration to the Brazilian government and the global community to adopt urgent measures to protect Indigenous territories.

In January, recently-elected president Luiz Inácio Lula da Silva declared a health emergency in the Yanomami Indigenous land, where the invasion of illegal miners had increased deforestation, contaminated rivers with mercury, hunted animals part of the Indigenous diet and brought severe cases of malaria, pneumonia, and malnutrition. Led by IBAMA, Brazil’s environmental agency, the so-called Yanomami Operation is expelling illegal miners from the Indigenous territory and, to a lesser extent, from others.

Currently, Indigenous leaders in the country expect the president to sign the demarcation of 14 Indigenous territories, including those in the Amazon, this month in April.

Part of a Yanomami community.
Part of a Yanomami community in March, 2022. Image by Carsten ten Brink via Flickr (CC BY-NC-ND 2.0).

According to Funai, Brazil’s Indigenous affairs agency, the country has a total of 764 Indigenous territories, of which 483 are ratified and 281 are awaiting the official demarcation process.

“The original peoples arrived in the Amazon 12,000 years ago and kept the forest standing,” said Nobre. “As we approach the irreversible tipping point for the Amazon, we are still doing too little to protect it.”



Tang, W. & Arellano, A. Jr Investigating dominant characteristics of fires across the Amazon during 2005–2014 through satellite data synthesis of combustion signatures. J. Geophys. Res. Atmos. 122, 1224–45. (2017).

Akagi, S. K. et al. Emission factors for open and domestic biomass burning for use in atmospheric models. Atmos. Chem. Phys. 11, 4039–4072 (2011).

Apte, J. S., Marshall, J. D., Cohen, A. J. & Brauer, M. Addressing global mortality from ambient PM2.5. Environ. Sci. Technol. 49, 8057–66. (2015).


Banner image: 2019 and 2020 saw massive fires in the Brazilian Amazon, largely experts say due to Bolsonaro’s failed environmental policies. Image by Sergio Vale / Acre state.

Related listening from Mongabay’s podcast: We speak with National Geographic photographer Kiliii Yuyan about the value of traditional ecological knowledge (TEK) in protecting the world’s biodiversity and examples of TEK from Indigenous communities he’s visited. Listen here:

Blazing start to Amazon’s ‘fire season’ as burning hits August record

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