Would you believe us if we told you that, when you breathe in, you inhale thousands of microorganisms with every breath. Although this might sound scary, be assured that they are safe for your health. These airborne microorganisms, too small to see with the naked eye, consist of many different species. Who are they? Where do they come from? What do they do in the air? These are some of the questions that we answer in this article, although many questions about airborne microorganisms remain to be explored.
What Are Airborne Microorganisms?
Microorganisms are tiny—around 1 μm, which is one thousand times smaller than 1 mm. Each individual microorganism is made of a single cell. These cells do not come together to create a larger multicellular organism, like human and animal cells do. Microorganisms are found in every environment on Earth, including soils, oceans, ice, and air (Figure 1). The microorganisms that are found in the air are called airborne microorganisms, and they travel either on their own or attached to dust particles. Dust particles can come from things like car exhaust and industrial pollution. Airborne microorganisms have various shapes (round, rod-shaped, or string-shaped) and can include many different species of bacteria, fungi, and algae. Consider all the different species of birds, their various sizes, shapes, and ways of living—then imagine that the same is true for airborne microorganisms. Each day, you inhale up to 15,000 l of air, containing millions of airborne microorganisms belonging to thousands of different species.
Where Do Airborne Microorganisms Come From?
The tiny size of microorganisms makes them extremely light and susceptible to every gust of wind. The airborne microorganisms floating around you are in constant motion, which is affected by the weather conditions. During extreme weather, such as cyclones or sandstorms, airborne microorganisms can be transported thousands of kilometers in just a few days (for example, from the African desert to Central America) . At some point, the microorganisms that have been swept up into the air fall back down to the Earth’s surface, due to gravity. However, unlike a ball that is thrown up in the air, microorganisms fall at a relatively slow speed and can stay suspended in the air for quite some time. Airborne microorganisms are constantly falling to the Earth’s surface and being replaced by new organisms that arrive in the air (Figure 2). These new microorganisms come from the soil, the sea, and the plants on Earth’s surface. A microorganism living in the sea can become airborne through a process called aerosolization. Aerosolization occurs when solid particles become suspended in the air and, in the sea, this can happen when air bubbles in the water are released into the air. These microorganisms cannot fly on their own—like a floating feather, they are held aloft and transported by the air.
The number and diversity of airborne microorganisms depend on the geographical location [2, 3]. Thus, depending on where you are located on the planet, you might inhale different numbers and kinds of microorganisms (Figure 1). The concentration of airborne microorganisms depends on both human activity and the number of microorganisms in the environment. If you live in a big city where there is a lot of human activity (lots of cars and industries, for example), the air will be filled with a high number of airborne microorganisms that come from dust and soil. However, if you live in the frozen region of Greenland, you will probably inhale many fewer microorganisms, since the snow and ice that surround you contain fewer microorganisms than do soils, farms, or cities.
What Do Airborne Microorganisms Do?
We still know little about what airborne microorganisms do. Each species has its own lifestyle. It has been suggested that some species help in the formation of clouds and rain . Airborne microorganisms might transform the chemical compounds around them and thus change the air quality. The air is composed of gases (oxygen, carbon dioxide, nitrogen), airborne microorganisms, and dust, but it also contains a multitude of very tiny chemical compounds that are like floating food for airborne microorganisms. Like every animal, microorganisms consume food and produce wastes, and these activities can change the chemical composition of the air. While we suspect that airborne microorganisms can eat while they are in the air, we do not know if they reproduce in the air. The air is a stressful environment, with harsh chemical and physical conditions. For example, high levels of solar UV radiation and extremely cold or hot temperatures are generally not good for living organisms, including most microorganisms (Figure 2). However, microorganisms are much more resistant to extreme environmental conditions than humans are, and they have tools to protect themselves. For example, the cell membranes of some microorganisms contain pigments called carotenoids, which act like a shield against UV radiation–similar to the pigment called melanin found in human skin. Other microorganisms have antifreeze proteins that can protect them from extremely cold temperatures. Under harsh conditions, some microorganisms can become dormant (go into hibernation) and wait for better conditions. Despite these protective measures, it is not clear how active microorganisms really are in the air.
How Do We Study Airborne Organisms?
Evaluating the number and diversity of microorganisms floating in the air requires specific tools. First, we need to catch airborne microorganisms and lock them up in a container that will be brought back to the laboratory (Figure 3). To collect lots of microorganisms in a container, we use a machine that works like a vacuum cleaner, sucking in the air and catching airborne microorganisms on a filter or in a tube . This is similar to vacuuming your house: you suck up the dust and it is collected in a container found inside the vacuum. Once this container is brought to the laboratory, equipment is used to extract and read the DNA of the microorganisms. The DNA is like an identity card that allows us to count how many airborne microorganisms there are and identify the species they belong to. To identify them, the DNA from airborne microorganisms are compared to a library of DNA from known microorganisms.
What About Coronavirus?
Airborne microorganisms have always existed, and you have been inhaling these tiny living things since you were born. Even though airborne microorganisms consist of bacteria, fungi, and algae, most of the time they do not make us sick. Airborne microorganisms are mostly safe for humans. You might be thinking, “But the SARS-CoV-2 corona virus that causes COVID-19 is found in the air, and it can cause disease in humans, cannot it?” You are right, the coronavirus can be found in the air, as can many other viruses. Viruses are not made of cells, so they are generally not considered to be microorganisms. While recent studies confirmed the potential transmission of the coronavirus through the air, scientists believe that the coronavirus is mostly transmitted between humans through tiny droplets of saliva that are projected into the air when infected people talk or sneeze. These saliva microdroplets disappear rapidly from the air, especially outdoors.
No matter where you are in the world, from the North Pole to the South Pole, there are millions of microorganisms in the air, which we breathe in with every breath. While invisible, they are alive and in constant movement, due to wind and air. Airborne microorganisms consist of thousands of different species, most of which remain largely unknown. They could perform many functions in the air, but we still do not understand these functions. Airborne microorganisms require further studies, and these studies promise to be extremely interesting, since there are many things we still do not understand. The exploration of airborne microorganisms has only just begun, and it is likely to lead to exciting discoveries in the areas of meteorology, air chemistry and human health!
Microorganisms: ↑ Microscopic organisms including bacteria, fungi, and algae.
Aerosolization: ↑ The process by which particles on the ground or in the water become suspended in the air.
UV Radiation: ↑ Ultraviolet (UV) radiation is a form of non-ionizing radiation that is emitted by the sun.
DNA: ↑ The genetic material found in each cell that is passed on to offspring from their parents and determines an organism’s characteristics.
Conflict of Interest
NT was employed by company N-ArT, Nice.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
 ↑ Griffin, D., Gonzalez-Martin, C., Hoose, C., and Smith, D. 2017. “Global-scale atmospheric dispersion of microorganisms,” in Microbiology of Aerosols, eds A.-M. Delort and P. Amato. doi: 10.1002/9781119132318.ch2c
 ↑ Tignat-Perrier, R., Dommergue, A., Thollot, A., Keuschnig, C., Magand, O., Vogel, T. M., et al. 2019. Global airborne microbial communities controlled by surrounding landscapes and wind conditions. Sci. Rep. 9:14441. doi: 10.1038/s41598-019-51073-4
 ↑ Tignat-Perrier, R., Dommergue, A., Thollot, A., Magand, O., Amato, P., Joly, M., et al. 2020. Seasonal shift in airborne microbial communities. Sci. Total Environ. 716:137129. doi: 10.1016/j.scitotenv.2020.137129
 ↑ Amato, P. 2012. Clouds provide atmospheric oases for microbes: estimated to total 1019 cells, microorganisms in clouds appear sufficient to affect physicochemical processes in the global atmosphere. Microbe Magazine. 7:119–23. doi: 10.1128/microbe.7.119.1
 ↑ Dommergue, A., Amato, P., Tignat-Perrier, R., Magand, O., Thollot, A., Joly, M., et al. 2019. Methods to investigate the global atmospheric microbiome. Front. Microbiol. 10:243. doi: 10.3389/fmicb.2019.00243