Air pollution is the main responsible for environmental quality worsening in many cities all over the world, with adverse outcomes on people’s health. According to the World Health Organization (WHO), more than 80% of people living in the urban context are subjected to air quality levels above the emission limits regarding air pollution.

The primary atmospheric pollutants are carbon monoxide (CO), particulate matter (PM), nitrogen oxides (NOx), volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), ozone (O₃), and sulfur dioxide (SO₂). All these pollutants are emitted by the processes of industrialization and urbanization.

Air pollution has severe damages on people's health, which can range from headaches and anxiety to cancer and chronic heart and respiratory diseases. But the health of people is not the only one that is affected by air pollution, also that of forest ecosystems.

Forest decline is as an episodic event characterized by the presence of symptoms such as reduced growth, shortened internodes, root necrosis, premature fall coloring, yellowing and loss of foliage, dieback of twigs and branches generally beginning in the upper crown, sprouting from adventitious buds, increased prevalence and pathogenicity of root decay, simplification of structural diversity, biotic impoverishment, reduced species diversity and reduced forest stands dynamics. The data available to date support the thesis that new types of forest decay are directly or indirectly related to ozone concentration and possibly sulfur dioxide. Many studies suggested that air pollution is linked with forest decline of the world.

These pollutants can be exemplified under the following scheme: humans generate emissions of sulfur dioxide and nitrogen oxides by increasing their concentration in the atmosphere; the action of the sun's rays cause the formation of secondary pollutants such as ozone; after some time (hours or months) these pollutants are transferred to terrestrial or aquatic ecosystems, by dry or wet deposition mechanisms. Meanwhile, a complex series of oxidations, mainly in the wet phase (e,g. rain) increases the amount of acidification within the atmosphere.

There is a very well studied case in Mexico. The forests of Abies religiosa are found at more than 2700m above sea level on the northeast and north slopes of the mountains of the southern Valley of Mexico, they were until a few years ago very dense forests, offering many places of recreation for hundreds of thousands population. From Mexico City. Unexpectedly, a few years ago there has been an acute process of decline that had not been seen before; hundreds of thousands of trees have died since the first sign of decline was recorded. The studies carried out indicate that the decline of these forests was mainly caused by the enormous concentration of atmospheric pollutants due to its proximity to Mexico City, where large amounts of particles are emitted every day.

Forest ecosystems are important for mitigating climate change and conserving biodiversity. Without these, life is at risk. It is urgent to take measures to reduce air pollution. Among what we can do is:

 - Save energy

 - Use public transportation, bike, or walk whenever possible

 - Mulch or compost leaves and organic waste

 - Reduce wastes

 - Participate in reforestations.

All actions matter.

Bibliography:

Cibrián T, D. 1989. Air pollution and forest decline near Mexico City. Environmental Monitoring and Assessment 12(1):49–58.

Sofia, D; Gioiella, F; Lotrecchiano, N; & Giuliano, A. 2020. Mitigation strategies for reducing air pollution. Environmental Science and Pollution Research.

Woo, S. Y. 2009. Forest decline of the world: A linkage with air pollution and global warming. African Journal of Biotechnology 8(25): 7409-7414.