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Just like in our eyecare field, SARS-CoV-2 knowledge is evolving ‘as we speak’. The newest kid on the lock are aerosols. According to Shelly Miller, Professor of Mechanical andEnvironmental Engineering, an aerosol is a clump of small liquid or solid particles floating in the air. They are everywhere in the environment and can be made of anything small enough to float, like smoke, water or coronavirus-carrying saliva. When a person coughs, talks or breathes, they throw anywhere between 900 to 300,000 liquid particles from their mouth.


A cough can send aerosols traveling at speeds up to 60 mph or almost 100 km/h. Size of the particle and air currents affect how long they will stay in the air. In a still room, tiny particles like smoke can stay airborne for up to eight hours. Larger particles fall out of the air more quickly and land on surfaces after a few minutes. The new coronavirus SARS-CoV-2 itself is tiny, about 0.1 microns in diameter. The aerosol or droplet is the vehicle for it to infect the raspatory system. The size of these vehicles range from
microscopic – a thousandth the width of a hair – up to the size of a grain of fine beach sand.

In essence, droplets can be defined still in millimeters, although in decimals of that, with a cutoff of 0.1mm size particles (or 100 microns if you want). Aerosols produced by people when they breathe, talk and cough are generally between about 0.7 microns to around 10 microns – invisible to the naked eye and easily able to float in air. It appears that the new nomenclature in SARS-CoV-2 land is aerosols versus droplets, and that means we have to think in microns instead of millimeters.

Microns is the New Millimeters

This is where the ‘Eyeing Dutchman’ sees similarities and overlap with our contact lens field. Up until recently, everything was marked in ‘millimeters of curvature’. This is a heritage from good old cornea GP lenses, where it actually made sense to ‘go flatter’ than the corneal curve. With the transition to soft lenses, the historical mistake was made to continue this.

It wasn’t until scleral lenses became popular again, that we started to think in height and microns: height of the ocular surface and of the lens to match that. Also, the clearance behind the lens was defined in microns, with a given thickness of the lens (in microns indeed) to provide enough oxygen to the cornea. We should also allow for about 100 microns of ‘Sinking’, and changes in scleral lens fit are typically defined in microns – like 200 micron increments in a trial set for instance.

While I am fully aware that the govern bodies around the world and ISO standard dictate to have our lenses marked in millimeters, I personally would love to move into measuring and thinking in microns in our contact lens field. A clearance of 75 microns just sounds more precise, accurate and more important than if it was put as 0.075mm. The significance of that simply seems smaller, while obviously it is the same thing. It would also help to explain to students for instance the distinction between ‘millimeters of
curvature’ versus ‘microns of height’ to introduce them in the new nomenclature.

The Flying Dutchman

This is a legendary ghost ship that can never make port and is doomed to sail the oceans forever. I don’t want to over-dramatize things. But I won’t rest and I will ‘sail the oceans forever’ until we have fully embraced height as a measure for fitting our lenses, ideally in microns if you ask me. I’ll keep an eye out on you. The Eyeing Dutchman is watching you.

Eef@online_The Eyeing Dutchman

This Eef@online series is kindly supported by an educational grant from Contamac
“With the transition to soft lenses, the historical mistake was made to continue to think and work in millimeters of curvature.”

In the series Eef@online articles: