Disinfecting Sprays and Droplet Size

Call me nuts.

Early March, I was still living on the road across the country. I knew COVID-19 was there. Having worked in a BSL-2 (Biosafety Level 2) lab for years, I thought I had a good handle over it. I was prepared with 70% IPA, then I bought a spray bottle in the beauty section at a grocery store. When I arrived at a new hotel room, I sprayed IPA over important surfaces, but apparently the spray bottle was meant for spraying skincare mists. A great portion of IPA droplets went to the air rather than the surfaces. The whole room smelled very bad yet not a lot of surfaces were covered… It got me interested in sprays.

Disinfecting Sprays is a Soaring Market

“Disinfecting sprays surged in U.S. sales by 519% during the week ending March 14 compared to the same period last year” In addition to the consumer market, disinfecting sprays are key to preventing hospital acquired infections (HAI). “In American hospitals alone, the Centers for Disease Control (CDC) estimates that HAIs account for an estimated 1.7 million infections and 99,000 associated deaths each year”. In fact, state health departments in Ohio and Minnesota are reporting that up to 20 percent of those infected are health care professionals, a number in line with Italy and other hard-hit regions of the world.

Sprays and Droplet Size

What makes a good disinfecting spray?

Droplet size is one of the most important parameters that determine the efficacy and spray drift of disinfecting sprays. Generally speaking, smaller droplets have higher efficacy but easier to drift. A study has shown that a droplet size range of 10μm32<25μm, where D₃₂ is the Sauter Mean Diameter (SMD), is suitable for healthcare applications[1]. With this size range, the spray was tested to give a good even coverage area; the droplets are fast evaporating, and have no excessive drifts. There is, therefore, no need to wipe with a cloth after spraying. Indeed, the cloth can be a problem in the conventional spray and cloth system as the fibers within a cloth usually hide germs [1]. However, most commercial sprays tend to have larger droplet sizes with a much wider size distribution.[https://patents.google.com/patent/WO2012156170A1/en] [https://patents.google.com/patent/US9333523]

While droplets too large are lower in efficiency and tend to be messier, droplets which are too small also arise, leading to health and safety concerns due to their tendency to get into the lungs. My experience of spraying IPA in hotels has reminded me that not all pumps are meant to spray chemicals with potential hazards, because, of course, the majority of us do not want a lungful of disinfecting fluids…… In fact, inhalation risk assessment has become mandatory to spray products in official regulations (e.g. 75/324/EEC, 1223/2009/EC, 1907/2006/EC). Such intensified requirements have led to a shortage of pumps up to standards. Together with the need for “better” pumps, there is the increasing need to test those pumps for inhalable risk, not to mention now with the Covid-19 situation.

How Spraytec can help with Droplet/Aerosol Sizing

A disinfecting spray produces droplets of a wide range. Both the average size and the size distribution are important. Especially for inhalation risk evaluation, the percentage of droplets less than 10 μm in size needs to be rigorously controlled. It is essential to have a statistical analysis for more efficient product development. Here we have an application note for more technical information

Malvern Panalytical’s Spraytec uses laser diffraction to measure spray particle and spray droplet size distributions in real-time. And you’ve got our support. Contact us and we will have innovative ways to perform demos at your site, or remotely.

Also, check it out here on how we used Spraytec for droplet sizing in coughs and sneezes.

References

[1] Nasr, G., Whitehead, A. and Yule, A. (2012) “Fine Sprays for Disinfection within Healthcare”, The International Journal of Multiphysics, 6(2), pp. 149-166. doi: 10.1260/1750-9548.6.2.149.