Aerobiology: The Air Is Alive – And Not In A Good Way!

Posted on | September 12, 2023 | 6 Comments

Mike Magee

When Paul Crutzen and his band of happy meteorologic warriors launched the Anthropocene Epoch in 2000, their guiding star was to create a “safe operating space” for humans on the planet Earth. In service of this goal, they identified nine “planetary boundaries” (measures of planetary health) as planning guideposts.

Number one, familiar to all, was Climate Change. It’s measure was atmospheric CO2 levels less than 350 ppm (parts per million).  How are we doing on that? Well, by any measure, not too well. This week’s headlines tell us that 2023 has had more weather related death and destruction than ever before, and the CO2 measure for 2023 now sits at 416.5 ppm.

Of course, this is not news. In fact, you’d have to be living under a rock to be unaware of the causal relationship between burning fossil fuels, rising atmospheric CO2 and weather related catastrophic wind, rain, and flooding from Libya to Leominster, MA. But what may catch many off guard is the news of what is traveling on the wings of particulate matter (including rain drops) fanned by these agitated atmospheric currents.

Welcome to the world of Aerobiology – the “study of aerosols with a biological origin.” Crutzen’s team listed it by another term – “Atmospheric Aerosol Loading” – as the #9 Planetary Boundary (PB) in 2015. A few weeks ago, The Lancet shined an unwelcome light on the issue with data suggesting that wild winds laden with microbes hitching on particulate matter appear to play a role in spreading drug resistant bacteria and fungi around the globe.

The Institute of Environmental Geosciences, Grenoble Alpes University, published a comprehensive review of PB #9 in 2020 titled “Microbial Ecology of the Planetary Boundary Layer.” What were their findings?

A Definition: “Aerobiology is a growing research area that covers the study of aerosols of a biological origin (i.e., bioaerosols) suspended in the atmosphere, from the air that directly surrounds us (both indoors and outdoors) to space by going through the different atmospheric layers.”

An Unhealthy Brew: “Bioaerosols include plant debris, pollen, microorganisms (bacteria, fungi, viruses, protozoans, etc.) as well as biological secretions which are mainly emitted by natural (forests, oceans, deserts, etc.) and urbanized Earth surfaces (agricultural fields, waste water treatment plants, cities, etc.) at different emission rates.”

Disease Focus: “Airborne microorganisms, especially bacteria, archaea, and fungi, are of particular interest as they represent living and potentially metabolically active cells light enough to be lifted high in the atmosphere by upward airflow.”

High Flyers: “During extreme meteorological events such as volcano eruptions and dust storms, sand-dust associated microorganisms can be ejected tens of kilometers high in the atmosphere before landing back on the Earth’s surface thousands of kilometers away. Microorganisms from the Bacillus and Micrococcus genera are commonly recovered from the stratosphere.”

Low Flyers: Particulate matter also travels in the lowest atmospheric layer – the troposphere. In addition to transporting microbes, troposphere particulates play a role in “meteorologic processes such as cloud formation and precipitation, atmospheric chemistry, and air quality.”

Growing Concern: “The capacity of microorganisms to be transported through the air has raised concern about the role airborne microorganisms might play in public health with the potential dissemination of plant and human pathogens as well as allergens.”

Does What Goes Up Come Down?: “The vertical gradient in microbial concentration suggests that microbial cell fluxes might be upward in the atmosphere.” (moving upward into the stratosphere). 

But not in weather like this: “Exceptions to the rule might occur during extreme meteorological events such as volcano eruptions, hurricanes, and sand dust storms. In the latter case, microorganisms associated to large particulate matter, such as macroscopic sand dust, could be lifted high in the troposphere, travel along global air masses over thousands of kilometers then settle back to the Earth’s surface due to gravity, precipitation, and atmospheric circulation.”

The air is alive: “Airborne microbial cells exist mainly as aggregates or attached to particulate matter (size range from less than one nanometer up to hundreds of micrometers like sand dust), while airborne fungi exist mainly as single spores. Microbial cells entering freely in the atmosphere can attach to existing particulate matter or other microbial cells. Conversely, particle-attached microbial cells can detach from their support in the air.”

Size matters: “15% of cultivable airborne bacterial cells were on particles <2.1 µm (size) and 25% on particles >7.2 µm, and that cultivable airborne fungal spores and cells were mainly distributed on particles between 1 and 3.2 µm (median-based values) on average in outdoor air.”

Travel time: “Within the planetary boundary layer, airborne microorganisms might have a residence time of a few days before returning to the Earth’s surface due to gravity or precipitation (model assuming that microbial cells behave like non biological aerosols). In the free troposphere, their residence time might be several days during which they might be transported over long distances.”

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