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Notes from the Stability Testing Laboratory – The Weather Girl.

March 2, 2021

I’m sure that many of you have, in the past, experienced the ravaging effects of hotness on your cosmetic products. I wrote about an experience I had a while back here and have since been mindful to remind my clients of the particular attraction that dark coloured packaging has to heat vs light. If that sentence has just had you going ‘wait what? Surely it doesn’t really matter what colour the packet is, just don’t leave your cosmetics out in the searing hot sun’ but actually it does matter. I am going off on a tangent a little to what I was going to write so I will be quick but basically the darker the packaging the faster it heats up and the slower it is to cool down.

While it is true that given enough time a product stored inside identical black and white packaging and exposed to the same amount of heat energy will eventually reach the same internal temperature. However the black pack will spend longer being hot than the white product. I would have hypothesised that without testing it as to me it seems fairly logical but about 3 years ago I had the chance to actually perform a stability protocol on a product to quantify just this! I can’t share the whole report here as it was confidential but I can say that the product stored in black packaging showed sighs of oxidation sooner and to a stronger degree than the product in white packaging at the same time point. I can’t remember the exact difference in heating curve but know the black pack was between 10-30% slower cooling down than the white pack which was significant enough to warrant investing in layered tubes that insulated the product better.

But today I want to talk about another weather phenomenon and that’s wetness.

Again most people can probably remember a day when they felt generally hotter and more irritated by the weather than the temperature should have warranted, often that’s due to humidity. Humidity is a term used to describe how much moisture is in the air and when the moisture levels go up, we tend to feel hotter at lower temperatures due to our sweat-cooling-mechanism not working so well. What you may not be aware of (or may not have thought about before) is that the air can hold different amounts of moisture at different temperatures.

There are two common ways to measure humidity – relative and absolute. Relative humidity is the percentage figure you often hear on the weather broadcast, absolute humidity is the actual amount of water in grams per meter cubed that is present. Once you know the maximum amount of water air can hold at a certain temperature you can calculate the amount present at any given relative humidity as I’ve done in the table below:

What this rather dry and boring table tells me in an instant is that at 20-25C, the ‘normal’ recommended storage temperature for a cosmetic, your products may be experiencing anything from 1.7grams of water per cubic meter to 17.3grams of water per cubic meter. What, you might ask, has that to do with anything given that most cosmetics are lidded and therefore won’t get wet? Well that depends doesn’t it…

Moisture permeability of packaging as supplied and in-use.

It’s rare for general cosmetic packaging to be completely air tight and in-use lapses of concentration or even difficult-to-manipulate closures can exacerbate the problem. I don’t have great fine motor skills (which is pretty annoying for my chemistry work) and in my day-to-day life I can struggle finding the thread on lids which often means the lids are either shoved on by-passing their natural thread dynamic or they are only half put on. Flip top lids are well known for flipping fully or at least half open and we all know that pumps are relatively tightly fit when we first buy a product but you have to unthread them to access the stuff thereby leaving a permanent air access point. Plastics and glass bottles and jars are not entirely air-proof either and will leak gradually over time no matter how diligently the lids are put back on a product.

This article looks at HDPE bottles and their ability to protect medicines from moisture and found moisture permeabilities between 12.57 – 149.95mg per day per litre of product. For the purposes of this exercise I’ll assume the liquid in the container was pure water which basically means the bottled water would gain between 0.1- 1.26% weight over a 12 week period when stored under equivalent conditions (75% humidity, 23C). To put this into an applied context, when I’m doing my stability testing of cosmetics, I fail anything that has a weight gain or loss of more than 1% over the 12 week test period. Products can gain weight from water taken in from the environment and can lose weight through evaporation so weight change is a very common phenomenon.

Exchanges between the product and the atmosphere increase the likelihood of preservative and oxidative failure of a product ahead of what you would normally expect. Sometimes oxidation happens first and in the worst case scenario the oxidative changes destabilises the product, reducing preservative efficacy which in turn leads to microbial failure. But this doesn’t always happen and some oxidative changes are barely noticeable and may not even make that much difference to the efficacy of the product. Alternatively it may be the preservative or microbial status of the product that gives up first. Moisture gain onto a dry, anhydrous or water-in-oil product can be the cause of unexpected and unwelcome mould growth, especially in formulations that are particulate-rich and may be harbouring microbial spores. Fungi often doesn’t need much moisture for it to start growing leaving products looking (and maybe smelling) quite grim.

So, what do we do?

I write these blogs because I’m interested in investigating what I see, the patterns that show themselves once you have journeyed around the sun a few times while doing the same things. I am interested in what we can and can’t control either by design or by instruction. On that note I’d suggest that what you do (should you wish to) is think about this when you are formulating, designing, storing or using your cosmetics. Remember that while I framed this as weather, it’s really about the environment- weather being the state of the atmosphere and the environment (from a product environment perspective) being the changes that your product is exposed to. People often bring up the bathroom when they are talking to me, stating the high temperatures and humidity as the reason they put a preservative in their dry salt scrub. I remind them that under those conditions, the product will most likely ruin (in terms of physical form) sooner than turn microbially unstable. They seem unconvinced but I advise them to try – salt quickly dissolves in water as does sugar.. On that note I remind people that fridges are a wet environment, that while reducing heat exposure is something it may not benefit the product if it means the oxygen is more soluble in it than the air (this mostly impacts vegetable oil based products stored in the fridge).

The bottom line is the environment interacts with and sometimes changes cosmetic products and cosmetic packaging doesn’t necessarily stop it.

One Parting Thought – World Wide Weather.

When writing this I started wondering about different places and imagining different scenarios. Formulating for a global audience has never been an easy task, not least because of the weather. Hopefully this table will help you to appreciate the wide and varying conditions that a global formula has to cope with. It may also help explain why customers living in different cities experience the same formula differently.

In this table I have placed major cities in the table to reflect their average relative humidity and their average annual temperature. Mostly I’ve just used the mean average but for Darwin I’ve split the data into wet and dry season to show how that alters things. If you like, you could use this table along with the table above to help you compare the average amount of moisture the air in each of these cities can hold. You could then compare what 80% humidity in Botoga means in terms of water volume vs that for Shanghai or Hong Kong.

The number in brackets is the city elevation because (as if you need more to confuse things), elevation affects air pressure and air pressure changes can also affect product stability, not least emulsion stability. Generally speaking, average air pressure drops as you rise in elevation. At 2000 meters you have only 80% of the air pressure as is present at sea level. Less pressure may translate to more movement in a formula which may lead to faster emulsion separation or oleogel crystalisation/ leaking. So much to think about but so much fun to consider don’t you think?

I hope you’ve enjoyed reading a little more on this topic and I do hope it has encouraged you to take a closer look at, and maybe even record, how your ingredients, formulations and finished products change along with the weather / environment.

Amanda x

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