Most of the overwhelming amount of research on exposure to air pollution is focused on urban centers and on the role of fossil fuels as the most important source of atmospheric pollutants. However, approximately 3 billion people in the world are exposed to air pollution from biomass burning, originating from using wood or coal as cooking fuel in simple stoves, home heating with open fires, deforestation, and agricultural practices.

In particular, the Brazilian Amazon region contains world’s largest tropical forest and is considered, during the rainy season, one of the continental regions least affected by human activities. However, during the dry season, high concentrations of aerosol particles from biomass burning (mainly agricultural practices and deforestation) have been documented in this region. The combination of forest fires and human occupation has turned biomass burning into a serious public health threat. The majority of forest fires occur in the deforestation arc, a belt in the southern and western regions of the forest, directly impacting over 10 million people in the area. Many studies in the area have identified severe effects on human health, such as increased incidences of asthma, morbidity and mortality, mainly in the most vulnerable populations such as children and elderly.

… Although epidemiological studies on the effects of urban PM on human health are numerous, there are relatively few that focused on the impact of air pollution resulting from biomass burning. … The objective of the present study was to investigate these effects in depth and provide a thorough analysis of the toxic cellular and molecular effects of relevant concentrations of PM₁₀ resulting from Amazon biomass burning, on human lung. In fact, this manuscript is a sequel to an article published recently by our group characterizing in detail the chemical composition of the inhalable material whose health effects are investigated here.

—de Oliveira Alves et al.

The study was conducted under the supervision of Carlos Menck, a professor at the University of São Paulo’s Biomedical Science Institute (ICB-USP), and Silvia Regina Batistuzzo, a professor at the Federal University of Rio Grande do Norte (UFRN). The research group also included Paulo Saldiva, a professor at the University of São Paulo’s Medical School (FM-USP), and Paulo Artaxo, a professor at the same university’s Physics Institute (IF-USP), as well as scientists at the Federal University of Rio de Janeiro (UFRJ), Oswaldo Cruz Foundation (FIOCRUZ), and Washington University in Saint Louis in the United States.

The researchers first determined the concentration of pollutants to be used in the lab experiments designed to mimic the exposure suffered by people who live in the area of the deforestation arc.

Using mathematical models, the researchers calculated the human lung’s capacity to inhale particulate matter at the height of the burning season and the percentage of pollutants that is deposited in lung cells. Based on this theoretical mass, they determined the concentration levels to be tested using cultured cells.

The pollutants used in vitro were collected in a natural area near Porto Velho, Rondônia, during the burning season, which peaks in September and October. The filters were frozen shortly after the particulate matter was collected because the organic compounds found in the pollution plume are highly volatile.

The cultured cells treated with the solution were compared with a group of control cells, which received only the solvent used to extract pollutants from the filters. The aim was to confirm that any adverse effects observed were caused by the particulate matter and not by the solvent.

In the very first moments of exposure the lung cells began producing large amounts of pro-inflammatory molecules. Inflammation was followed by an increase in the release of reactive oxygen species (ROS), substances that cause oxidative stress. Large amounts of ROS cause damage to cellular structures.

To understand the pathways that were inducing oxidative stress, we analyzed the cell cycle and found that it was impaired by an increase in the expression of proteins such as P53 and P21. The cells stopped replicating, which suggested that DNA damage was occurring.

—de Oliveira Alves

The researchers performed specific tests to confirm genetic damage. Based on their observation of increased expression of the protein LC3 and other specific markers, they also found that the cells entered a process of autophagy whereby they degraded their own internal structures.

Whereas only 2% of control cells died from necrosis after 72 hours, in the culture treated with pollutants, cell mortality reached 33%. The death of large numbers of lung cells in a living organism can lead to breathing problems and even severe diseases such as pulmonary emphysema, noted Artaxo. The surviving cells suffered DNA damage, which may predispose them to the development of cancer in future.

Before beginning the experiment with cultured cells, the team had completed an analysis of the substances present in the particulate matter collected in the Amazon region. They identified the presence of several PAHs, many of which are known to be carcinogenic. The results of this analysis were published in 2015 in the journal Atmospheric Environment.

We observed that the most abundant PAH was retene. We therefore decided to repeat the experiment with the cells using this substance in isolation but at the same concentration as that found in the particulate matter. We observed that retene alone also induced DNA damage and cell death.

—de Oliveira Alves

Although retene is not emitted by the burning of fossil fuels, the main source of pollution in Brazil’s urban areas, the researchers stress that this compound can be found in the atmosphere in São Paulo and other cities, probably owing to the burning of sugarcane and other kinds of biomass on nearby farms.

We found no information on the toxicity of retene in the scientific literature. I hope our findings serve as an incentive for the compound to be better studied and for its environmental concentrations to be regulated by health organizations.

—de Oliveira Alves

Resources

• Nilmara de Oliveira Alves, Alexandre Teixeira Vessoni, Annabel Quinet, Rodrigo Soares Fortunato, Gustavo Satoru Kajitani, Milena Simões Peixoto, Sandra de Souza Hacon, Paulo Artaxo, Paulo Saldiva, Carlos Frederico Martins Menck & Silvia Regina Batistuzzo de Medeiros (2017) “Biomass burning in the Amazon region causes DNA damage and cell death in human lung cells” Scientific Reports 7, Article number: 10937 doi: 10.1038/s41598-017-11024-3