What are aerosols?

What are aerosols?

Imagine you heard from someone that you were surrounded by millions of microorganisms. At home, at work, in the gym, even in nature: Every day they are there. Sounds scary doesn’t it? These microorganisms that surround us are called airborne biological particles or just bioaerosols for short and are categorized under the general term aerosols. You’ve heard that word before, right? Just as a refresher, aerosols are generally defined as “colloidal systems of (liquid or solid) particles suspended in a gas (usually air).”

But what does that really mean?

Well, you come in contact with aerosols every day: deodorants, cigarette smoke, dust, and exhaust emissions. These are all aerosols we constantly encounter when we live in an urban environment. We call them aerosols of anthropogenic origin. Hardly anyplace in the world is free of anthropogenic aerosols, and we inhale them continuously. But aerosols also originate in nature: fog, sand, forest exudates, and sea spray are just a few examples. In nature, airborne dust consists of up to about 25 % biological particles; in urban and agriculturally dominated areas, this percentage is usually higher. 

In this text, we will focus on bioaerosols because they are the most interesting type of aerosols to those working in labs.

Bioaerosols

A bioaerosol is an aerosol comprising particles of variable biological origin. This can be fungal spores, pollen grains, endotoxins, or particles of animal dander. Bioaerosols are complex mixtures consisting of several components that can stem from simple organic molecules (dimensions in the nanometer range), viruses, bacteria and bacterial spores, mold spores and hyphae, pollen (with diameters as small as 100 micrometers), and animal and plant debris (of various sizes). It’s easy to imagine the kind of critical role these bioaerosols can play in your daily laboratory work! Aerosolization not only takes place through wind and spray and through breathing, speaking, coughing, and sneezing; it also occurs through lab activities such as pipetting, centrifuging, opening of ampoules, and shaking – just to name a few.

We differentiate among different sizes of aerosols

Droplets (aerosols larger than 100 micrometers) tend to land on the ground before they evaporate. These droplets can cover surfaces (like tables, instruments, etc.) and contaminate them. They therefore constitute a source of staff exposure to infections. 
Dust (10 to 100 micrometers) and droplet nuclei (smaller than 10 micrometers) are very small aerosols. Droplet nuclei consist of bacteria (or another biological agent) in a droplet. When the droplet fluid evaporates, the bacteria remain in a dried state. This is a form of bioaerosol. 
They are so small and light, they may remain suspended in the air for several hours. Also, air currents can widely disperse airborne droplet nuclei.

Generally speaking, the settling velocity (the number of microorganisms that settle on a petri dish in a given time) of aerosols can vary greatly and depends on the particle type. The greater the settling velocity, the lower the risk of exposure to the operator. This is because the bigger the particle, the greater is the settling velocity and vice versa.
This means bioaerosols can potentially expose personnel in two different ways. To understand this, consider this: The human respiratory tract is as big as a tennis court. With every breath, millions of particles come into contact with our respiratory tract. 
Some of the particles, especially the ones smaller than four micrometers, will gain access to lung tissue. And some can even reach our bloodstream. Ingestion and dermal contact is also possible with larger droplets that contaminate the surface of laboratory equipment and other surfaces in the lab.

Very common laboratory activities produce aerosols, and we only mention a few of them in this text. Often, lab activities are carried out without even thinking about aerosols, but around the world there are many documented cases of laboratory-acquired infections resulting from the production and inhalation of infectious aerosols. In your daily laboratory work, centrifugation is just one source of aerosols, and you must take adequate precautions when working with infectious or harmful substances.
During centrifugation, aerosols can escape during the high-speed spin process, for example, if microorganisms are not adequately contained within the centrifuge. This may occur if you spin uncapped samples or if a tube leaks or breaks. In the following, we will provide some advice on minimizing aerosol production while centrifuging.
As a first step, make sure to choose the right centrifuge vessel and consider what precautions to take. The rotor plays an important role in preventing formation of aerosols outside the centrifuge. For this reason, you should know how to clean and disinfect the centrifuge and its accessories after a run and should there be leakage.