The instruction nobody reads
Somewhere on the back of a sun protection pack, usually near the directions, is a line telling you to keep the product out of direct sunlight, or to avoid excessive heat, or both. It is one of the least prominent statements on the pack and one of the few that describes a real condition on everything else printed there.
The reason it is there is that the figure on the front and the durability marking on the back both assume a product that has been kept in a certain range. If it has not, neither statement describes the pack you are holding.
What heat does, as a mechanism
Three things change with temperature, and they compound.
Chemical change accelerates. Reaction rates generally rise with temperature. Ultraviolet filters are functional chemistry, and a formulation held warm for a long period is a formulation in which change proceeds faster than the stability testing assumed.
Emulsions destabilise. Most sun protection products are emulsions, and an emulsion is a metastable arrangement held together by design. Heat, and repeated cycling between warm and cool, works against that arrangement. Separation, coalescence and change in viscosity all follow.
The film changes. This is the part that connects to the number. The tested protection factor is a property of a film formed by a particular formulation spreading in a particular way. A product whose emulsion has partially separated does not spread the same way and does not form the same film. It may still contain the same filters and still not deliver the same performance.
That last point is the one worth carrying away. Degradation of a sun protection product is not only about ingredients breaking down. It is also about a formulation losing the physical behaviour that made the tested film possible.
Why there are no numbers here
Articles on this subject usually reach for a temperature threshold and a rate of loss. We are not able to source either to a standard we are prepared to cite, so we do not print them.
Stability testing is conducted under defined conditions and the results sit in the product information file, unpublished. Manufacturers hold data on their own formulations; that data is specific to those formulations and is not generalisable into a rule about sunscreen as a category.
What can be said without a figure is the direction and the mechanism, which is what matters for reading a label: warmer is worse, cycling is worse than a steady temperature, and the visible signs appear later than the change does.
| Change | Mechanism | Visible |
|---|---|---|
| Accelerated chemical change | Reaction rates rise with temperature | Not at first |
| Emulsion separation | A metastable arrangement destabilises | Yes, at later stages |
| Viscosity change | Structure of the formulation alters | Yes |
| Altered film formation | The product no longer spreads as designed | Sometimes, on application |
| Colour or odour change | Chemical change in the formulation | Yes |
| Loss of tested performance | Follows from the above | No, never |
Framework of this publication. It describes how published rules and guidance fit together and is not a measurement, a survey or a study.
The car, which is the specific case
The interior of a closed vehicle in sunlight reaches temperatures well above the surrounding air, because short wavelength radiation enters through the glazing, is absorbed by interior surfaces and is re-radiated at wavelengths the glass transmits less readily. It is an ordinary optical result and it is why a car interior behaves as it does.
A sun protection product lives in exactly the places where this happens: a glovebox, a door pocket, a boot, a bag left on a seat. And it lives there during the season when the effect is strongest, which is also the season when the product is in use.
The same applies to a bag on sand in direct sun, to a windowsill and to a cupboard against a sun-facing wall in a warm month. The pattern is that sun protection products are stored, by their nature, in warm places.
The dormant months
The other case is the opposite one, and it is less obvious. A pack bought in one summer and used again the next spends most of a year unopened somewhere.
Where that somewhere is a holiday bag in a loft, a car boot or a garage, the product experiences a long period of temperature cycling with nobody looking at it. The durability marking continues running throughout, and the assumption behind it, storage under appropriate conditions, is quietly not being met.
This interacts directly with the period after opening marking discussed in period after opening and expiry dates, because the marking measures elapsed time and not the conditions during it.
What you can and cannot see
Some changes are visible: separation into layers, a change in colour, a change in smell, a texture that has become thin, grainy or greasy, or a product that no longer spreads as it did. Any of those is a clear signal that the formulation is not what it was.
What is not visible is chemical change that has not yet produced a physical one. A product can look entirely normal and have been held at a temperature that accelerated change. Absence of a visible sign is not evidence that nothing happened.
This asymmetry is worth stating plainly, because the natural test people apply is a visual one, and the visual test only catches the later stages.
Cold, and the other direction
Attention in this subject falls almost entirely on heat, and the other direction deserves a paragraph because it acts through the same mechanism.
Emulsions are metastable arrangements, and freezing stresses them physically as well as chemically. Ice formation disrupts the structure, and a product that has frozen and thawed may separate or change texture permanently. Storage instructions on some products address this alongside heat for that reason.
More common than either extreme is cycling. A pack that warms in a bag during the day and cools overnight, repeatedly across a season, is subjected to a stress that neither a steady warm temperature nor a steady cool one applies. Stability testing programmes commonly include cycling protocols precisely because the effect is different from a static hold.
The general principle behind all of it is stability rather than temperature as such. What degrades a formulation is departure from the conditions its structure was designed for, in whichever direction, and the visible consequences arrive well after the change begins.
What the label is telling you
Read together, the storage instruction and the durability marking say something coherent: this product will perform as tested for this long, if you keep it as we told you.
That is a conditional statement, and the condition is on the back in small type while the performance is on the front in large type. It is the same asymmetry that runs through the whole of this publication, and in this instance the condition is one the reader actually controls.
Where the durability figure comes from
Since the marking depends on stability, it is worth stating in outline where the figure originates, without pretending to more detail than is public.
Stability testing holds samples of the finished product, in its intended packaging, under defined conditions over a period, and assesses them against agreed criteria: appearance, colour, odour, viscosity, pH, separation and, for a sun protection product, the performance the product is sold for. Accelerated conditions are commonly used alongside real time storage, on the basis that elevated temperature brings forward changes that would otherwise take longer to appear.
Compatibility with the pack is assessed alongside stability, because a formulation can interact with the container it sits in over time.
The output of all of that is a durability figure held in the product information file and expressed on the pack as a date or a period after opening. It is a projection from controlled conditions, which is precisely why the storage instruction accompanies it.
What to take from this
Heat accelerates chemical change and destabilises emulsions, and a formulation that has separated does not form the film that was tested. The durability marking assumes specified storage. We describe the mechanism and print no figures, because the data sits in unpublished files and is specific to individual formulations.