Heat, Hot Cars and Your Makeup Bag: What the Stability Data Says

By the Belle Vault Editorial Team · Sources checked September 30, 2026

The makeup bag rides on the back seat for a two-hour lunch stop in July. The lipstick comes out soft and slightly bent. The foundation looks fine, which is not the same thing as being fine.

The FDA's advice on this is short: don't leave cosmetics where they are exposed to heat, such as in a hot car, because heat can make preservatives break down and let bacteria and fungi grow faster [1]. What it doesn't say is how hot a car actually gets, or how that compares to the temperatures products are designed to survive.

Both of those have been measured. Put side by side, they explain why a parked car is harder on a beauty kit than an airplane, and why sunlight may matter even more than heat for some of what is inside.

How hot a parked car gets

Study one: moderate days, dark sedan

McLaren, Null and Quinn measured the inside of a dark sedan on 16 clear days with outdoor temperatures between 72°F and 96°F. Over an hour, the inside temperature rose by about 40°F on average, and 80% of that rise happened in the first 30 minutes. Cracking the windows made almost no difference [2].

Study two: hot days in Arizona, sun versus shade

Vanos and colleagues parked cars in Tempe, Arizona, on summer days with outdoor temperatures above 100°F. In the sun, the average cabin air reached 116°F after one hour, with dashboards averaging 157°F and seats 123°F. In the shade, cabin air was closer to 100°F, dashboards 118°F and seats 105°F [3].

The two studies measured different things under different weather, so they don't give one number. They agree on the pattern: most of the heat arrives in the first half hour, and surfaces in direct sun, like the dashboard or a seat, run far hotter than the air.

What products are designed to survive

Cosmetics makers test stability by storing products warm for weeks or months. The industry guideline from Cosmetics Europe describes tests "often performed at 37°C, 40°C or 45°C during 1, 2, 3… months" [4]. That is 99°F to 113°F.

So a shaded car on a hot day sits near the top of that range, and a sunny one is above it within the hour. A dashboard in the sun is about 44°F past the hottest standard test condition.

Those numbers are easy to over-read. A stability test holds a product warm for months. A car heats it for hours. One afternoon at 116°F is not the same exposure as three months at 104°F, and the guideline itself says companies use the warm results to predict real shelf life, then confirm with real-time testing [4]. What the comparison does say is that a sunny car is outside the conditions products are routinely tested for.

The counterintuitive part: light may be doing more damage than heat

Heat gets the attention, but one study of active ingredients points somewhere else. Temova Rakuša and colleagues tracked retinoids, the vitamin A ingredients in many night creams and serums, in 12 commercial products for six months. At room temperature, 25°C, they lost between 0% and 80% of their retinoid content. At 40°C, they lost between 40% and 100%. And light degraded them more than temperature did [5].

The spread matters as much as the numbers. Some products lost almost nothing at room temperature; others lost most of their retinoid without any heat at all. So this is not a rule about every jar. It is a reason to treat a sunny dashboard as a double exposure: heat and light at once.

Sunscreen and expiration dates

Sunscreen is regulated as a drug in the U.S., which is why it carries an expiration date. The FDA recommends not exposing sunscreen containers to direct sun, and says a sunscreen without an expiration date should be treated as expired three years after purchase [6]. Most other makeup has no required date at all. The FDA notes there is no U.S. law requiring cosmetics to carry one [1]. In the EU, products that last 30 months or less must show a best-before date, and longer-lasting ones must show a period-after-opening symbol, the small open jar with a number of months [7].

What to do with this

1. Take the kit with you, or put it in the trunk in the shade. The first 30 minutes do most of the heating, so a short stop is not a safe stop [2].
2. Keep it out of direct sun and off the dashboard. Surfaces in the sun run 40°F or more hotter than the air, and light degrades some actives faster than heat [3][5].
3. Treat separation, a new smell or a color change as a warning sign. The FDA lists all three as effects of heat, sunlight and air [1]. It sets no rule for when to discard a changed product, so replacing it is the cautious choice.

Heat is also hard on the bag itself; see why makeup bags lose their shape and peel. For a closed, structured bag that keeps products out of the light, see the Belle Vault Travel Makeup Bag.

Frequently asked questions

Can makeup go bad in a hot car?

It can. The FDA advises against leaving cosmetics in a hot car because heat can make preservatives break down and let bacteria and fungi grow faster. In a 2018 Arizona study, cabin air in a car parked in the sun reached 116°F in an hour, above the 99°F to 113°F range cosmetics are commonly stability-tested at.

How hot does it get inside a parked car?

In a study of 16 days with outdoor temperatures from 72°F to 96°F, a dark sedan's inside temperature rose about 40°F in an hour, 80% of it in the first 30 minutes. In Arizona summer heat, cabin air reached 116°F in sun and about 100°F in shade, with sunlit dashboards averaging 157°F.

Does lipstick melt in a hot car?

It can soften, bend or sweat. Lipsticks are wax-based, and a sunny car's seats and dashboard reach 123°F to 157°F within an hour in hot weather. Keep lipstick out of direct sun and off the dashboard, and let a softened one cool fully before using it.

Is makeup still safe after being left in the heat?

It depends on the product and the exposure, and a quick look can't prove safety. The FDA notes that heat, sunlight and air can change a product's color, texture and smell, and make emulsions separate, but sets no rule for discarding. If something changed after a hot day, replacing it is the cautious choice; for skin or eye concerns, ask a dermatologist.

Do cosmetics have expiration dates?

Not necessarily in the U.S. The FDA says no law requires cosmetics to carry one, though sunscreen, regulated as a drug, must. The FDA says sunscreen without a date should be treated as expired three years after purchase. In the EU, products lasting 30 months or less need a best-before date; others show a period-after-opening symbol.

Does sunlight damage skincare?

For some ingredients, yes. A study of 12 retinoid products found that light degraded retinoids more than heat did, with losses at 40°C ranging from 40% to 100% over six months. Results varied by formula. Keeping products in their original, often opaque, packaging and out of direct sun reduces that exposure.

How we sourced this

Car temperatures come from two independent studies, reported separately because their conditions differ. The stability-testing range comes from the industry guideline; retinoid losses come from one peer-reviewed study of 12 products, and we note that its results varied by formula. Regulatory statements come from the FDA and the EU cosmetics regulation. We did not find a published measurement of airline cargo-hold temperatures, so the article does not make claims about them.

1. U.S. Food and Drug Administration. Shelf life and expiration dating of cosmetics. fda.gov
2. McLaren, C., Null, J., & Quinn, J. (2005). Heat stress from enclosed vehicles: moderate ambient temperatures cause significant temperature rise in enclosed vehicles. Pediatrics, 116(1), e109–e112. publications.aap.org
3. Vanos, J. K., Middel, A., Poletti, M. N., & Selover, N. J. (2018). Evaluating the impact of solar radiation on pediatric heat balance within enclosed, hot vehicles. Temperature, 5(3). Summary via Arizona State University. news.asu.edu
4. Cosmetics Europe / CTFA (2004). Guidelines on stability testing of cosmetic products. Industry guideline. cosmeticseurope.eu
5. Temova Rakuša, Ž., Škufca, P., Kristl, A., & Roškar, R. (2021). Retinoid stability and degradation kinetics in commercial cosmetic products. Journal of Cosmetic Dermatology, 20(7), 2350–2358. onlinelibrary.wiley.com
6. U.S. Food and Drug Administration. Sunscreen: how to help protect your skin from the sun. fda.gov
7. Regulation (EC) No 1223/2009 on cosmetic products, Article 19. legislation.gov.uk