The science of spitting in your mask

Spit in your mask? Not any more, according to Covid rules. Michelle Haywood looks at the science of mask fogging and the various solutions.

Following on from the outbreak of SARS-CoV-2 in early 2020, divers were forced to stay home. Once we were finally allowed to resume diving there was a raft of infection control measures put in place to ensure that we weren’t creating transmission routes.

One of these measures affected one of the fundamental skills learned during training, how to spit into your mask. Now, I’m not claiming to be posh, but where I grew up spitting just wasn’t the done thing. When I learned to dive, I had to overcome my natural reluctance to gob in my mask. And I see the same aversion from some of my new dive students. One of them is so disinclined to expectorate, she licks her finger and rubs it over the glass as a compromise.

We all know why we spit, it’s to stop the steaming up of your mask. Misty masks are inevitable, as water vapour from your breath condenses on the cold inside of the glass lens. If we could magnify the surface of the mask and look at the glass really closely we would find millions of microscopic depressions. This uneven surface creates ideal conditions for the beads of moisture to settle on the lens.

Spit contains several proteins that act as surfactants, and it’s these proteins (rather than the remnants of your latte) that help stop the layer of mist forming inside the mask. Surfactant is a term that comes from the phrase surface active agent. They break the bonds between the water molecules, so instead of forming little beads of condensation, the water spreads out like a film across the lens. The water can’t form into droplets, so there’s no refraction of light from the rounded surface and therefore no misty effect. Instead, the film of water falls to the bottom of the mask.

New masks have a thin layer of silicon on the surface, which creates even more sites for water to bead. Preparing a new mask for use involves trying to remove this layer, either using toothpaste or a proprietary mask prep solution. Be aware that sunscreen on your fingers can recreate this ‘new mask’ effect and won’t be overcome with saliva. You’ll need to properly clean the mask if you’ve managed to get sunscreen on the lens.

Of course, spit isn’t the only option. There are several commercial defog products available, but they all work in the same way as spit, as a surfactant that prevents water beading. Although the bottles are small, they are fairly efficient so only a small amount is needed each time. Many require application to a dry mask and don’t need rinsing afterwards. Many divers use baby shampoo to achieve the same effect. Baby shampoo has milder cleansing agents (that’s the surfactant bit we are interested in) compared to regular shampoo. This is important, as adult shampoos with their sulphates will irritate your eyes.

One of the key aspects, whichever defog you choose, is to apply the surfactant to the dry surface of the mask. No defog solution works as well if the mask is already wet before it is applied. Water will already occupy some of those microscopic pits... and that will come back to haunt you. Even having one or two spots of water will lead to localised misting.

So, evolution has blessed us with a pretty endless supply of natural defog, but in the current climate spitting in your mask (and rinsing it in a communal bucket) is definitely not acceptable. There’s reasonable evidence that Covid doesn’t survive seawater terribly well, but as you spit you will be creating an aerosol of droplets, possibly containing viral particles.

Now we are so much more aware of how easily Covid spreads, why would you endanger those around you by insisting on spitting? Even cricket has had to work out how to stop bowlers spitting on the ball during matches. I can’t find any evidence that they have resorted to baby shampoo though!

This column article was originally published in SCUBA magazine, Issue 108 November 2020. For more membership benefits, visit bsac.com/benefits.

Images in this online version may have been substituted from the original images in SCUBA magazine due to usage rights.