I’ve always had reservations about the use of aerosol* based sunscreens since I had heard reports of people accidentally setting themselves on fire in the United States. Having seen an increased number of product performance complaints about aerosol based sunscreens, I had serious doubts about the ability for these products to achieve the claimed sun protection factor (SPF) and ability to prevent sunburn.
I have investigated the potential dangers of aerosol sunscreen using basic research and laboratory testing using an aerosol sunscreen that is currently on the market. While I don’t disagree with the convenience of the product and that it could encourage people to use sunscreen when they otherwise may not have and that there is a consumer demand for such products, the investigation show that my reservations were very much warranted. As I will now explain, while the products appear easy to use, they must be used with caution, so much so, that consumer should consider whether the ‘convenience’ of aerosol application is worth the risk associated with their use.
As previously noted, there have been instances in the United States where people have caught on fire after applying aerosol based sunscreen products, causing serious burns. This is not surprising given the flammability of the Hydrocarbon (Butane) propellant/gas and Ethanol solvents used in these products. The dangers are clearly stated on pack in the form of a flammable goods placard and other written warnings on the dangerous and how not to use the product.
The risk remains that applying sunscreens around BBQ’s, people who are smoking and other sources of spark or naked flame can cause the product and persons wearing the product to catch on fire . The irony being while in the process of attempting to reduce the risk of sunburn we have increased the risk of an actual burn to our skin.
Asphyxiation refers to breathing in a gas/propellant with a reduced amount of oxygen in the air which lowers oxygen concentration in the blood stream, potentially leading to unconsciousness. Again, despite warnings on packs advising that sunscreens must be applied in a well-ventilated area, there is a risk that the user will apply the sunscreen in an enclosed space, potentially leading to asphyxiation and unconsciousness.
No sunscreen should be left in a car which is in the sun (especially during summer) or exposed to direct sunlight or left anywhere where it may be exposed to temperatures over 30degC, as elevated temperatures will cause a degradation of the product and impact the claimed performance. The need to avoid high temperatures is even more important for aerosol sunscreens which are pressurized containers, as heat will cause the pressure within the can to increase and there is a risk of the aerosol explodin.
SPF testing and certification
The sun protection factor (SPF) of aerosol sunscreens aren’t tested and certified in the laboratory using the same application methods used for regular sunscreens and by us as consumers. The reason for this is one of practicality; the propellant/gas in the aerosol makes following the standard test method virtually impossible as the amount dispensed and applied to the skin cannot be accurately measured as the packaged product in its original form is so volatile, because of this, an alternative method is used. During laboratory testing, the volatile propellant/gas is removed from the aerosol can before testing starts, leaving only the liquid inside, this liquid is then tested in the same way as with a regular sunscreen. The propellant/gas accounts for around 40% of the contents of the aerosol, meaning 40% of product is lost during the application process with only 60% of the contents having the ability to reach our skin where it can provide the necessary protection.
The above is not a flaw in the test used to certify the SPF, however it does impact the products ability to achieve its stated SPF as a result of needing more sunscreen to be applied than would be thought necessary.
Following on from the above, when tested in the laboratory, the product is physically applied to human skin and ‘rubbed’ in, however the directions and the aerosol packaging require that the product be sprayed directly onto your skin. The spray application can affect the quality of the sunscreen layer that is formed and subsequently the quality of protection. The image below shows a close-up photo of the sunscreen after application using the sunscreen liquid rubbed on vs directly sprayed on from the aerosol sunscreen packaging, you can clearly see the difference in texture of the sunscreen and these differences could affect the quality of protection:
Loss during application
Because of the gas/propellant and the fine mist released during use, the amount of product that comes out of a can of aerosol sunscreen is not the same as what ends up on our skin. Most brands recommend spraying the product 10-15cm away from our skin and laboratory testing under ideal conditions (i.e. indoors – well ventilated area, no wind) using these guidelines found that only around 40% of the amount dispensed from the pack actually ends up on the skin. The loss of 60% of the product in application is now 20% above the gas/propellant loss outlined earlier where it was estimated 60% would reach the skin this additional loss is attributable to ‘overspray’. This loss in application would be worse if there were a breeze, if the area of application was small (such as a child’s arms and legs) or was being applied from a distance greater than the recommended 10-15cm.
Combining the amount of gas/propellant lost during application and the overspray, the total loss during application is significant at 60% of the amount in the aerosol, with only 40% reaching the skin where it can provide the necessary protection.
Change in pressure and amount applied
Over the life of an aerosol as the product is used, the amount of product/propellant in the can becomes less and this reduces the pressure within the aerosol. The pressure in the aerosol is responsible for the product being released during application, less pressure in the aerosol can results in the amount of product dispensed over a particular time becoming less. Some research found that the product would need to be sprayed 45% longer as the can approaches being empty to allow the same amount of product out of the can. Whilst it is reasonable for a user to take additional time to ensure proper application, given the sunscreen is transparent, it would be difficult for a person to understand that this would be necessary to ensure they achieve the claimed level of sunscreen protection (SPF).
Similarly, ambient temperatures can impact the aerosol pressure. Cooler temperatures reduce the pressure and the application amount, whilst warmer temperatures will increase the pressure and increase the application amount.
Understanding the application properties
To achieve the claimed SPF for regular sunscreen (non-aerosol), the average adult requires approximately 36g of product when being applied over the entire body, for the golfers out there; this is roughly the size of a golf ball in terms of volume, for the cooks, its 1.5-2 tablespoons. It is difficult enough for a person using a regular sunscreen lotion to understand what this means when sunscreens aren’t sold in golf balls and we don’t carry a set of measuring spoons. It’s even more difficult to comprehend when using an aerosol sunscreen and the dispensed product which is bordering on invisible!
I decided to try applying the sunscreen for myself to see how much sunscreen reaches my skin, weighing the aerosol before and after application, timing the application to get an idea of how long the process takes and applying what ‘felt’ appropriate. I spent a total of 2 minutes applying the aerosol sunscreen and calculated that roughly 35g of product was dispensed from the aerosol. Allowing for the gas/propellant and over-spray, I estimated 40% of the 35g would have reached my skin, meaning 14g of sunscreen reached my skin, which is just 35-40% of what I should be wearing to achieve the claimed level of sun protection (SPF)
So, should I use aerosol sunscreen?
Using the diagram above as a guide, an adult who would normally require 36g of product would need 90g from an aerosol sunscreen to obtain the 36g of product. The 90g required makes ~4.5 minutes of continuous spraying to achieve, 4.5 minutes spraying a flammable product which dramatically increases the likelihood of catching fire!
Below is the previous diagram showing the weight distribution during application to clarify.
This means that for the average adult applying sunscreen to their entire body, you should be getting only 2 applications from the average 175g can (2 applications x 90g = 180g, a fraction more than the 175g in the average aerosol).
Going back to my experiment where I applied 40% of the recommended amount of sunscreen onto the skin i.e. 14g of the 36g dispensed as a result of propellant/gas and over-spray, I calculated that the SPF50+ sunscreen (over 500 minutes of protection) was now only SPF5 (50 minutes of protection), less than 10% of the claimed protection.
The dramatic reduction in protection when combined with the increased risks of flammability and explosion, I know that I won’t be using aerosol based sunscreens, having said that however, aerosol sunscreens are safe to use when used correctly, so for those who prefer the convenience of using aerosols, please use this article as a guide to how best to achieve the most out of your favorite sunscreen and minimize the risk.
*Aerosol in the context of this article refers to aerosol spray/mist type packaging as distinct from other forms of aerosol, including bag-on-valve and bag-in-can where the propellant is separated from the product, as used for dispensing creams, lotions and gels which don’t have the same dangers with respect to flammability, asphyxiation, explosion, losses during application etc.