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Many websites will claim a variant of the following claim:

Do not buy a used car seat unless you can verify the age of the seat. There should be a manufacturer's label on the back or bottom of the seat that gives the manufacture date and/or specific expiration date. All car seats and boosters have expiration dates.

Or something like:

No, it doesn’t suddenly become illegal to use a car seat at 10 years and 1 day after it was made, and there won’t be a warrant out for your arrest. But we know that you’d do anything to keep your sweet babe safe, and that’s why it’s recommended that you replace your car seat once it expires.

Is there any research supporting this statement? I.e. companies buying a bunch of used car seats from Craigslist, putting them into a car crash test and then seeing diminished performance with respect to their original stated specifications. Or alternatively, independent companies buying brand new seats and comparing them in a car crash test to "expired" 10-year-old seats and seeing whether a statistically significant difference in apparent or likely injury rates emerges?

Update as requested: this question isn't asking "how do car manufacturers determine the expiration date" - this would be off-topic for Skeptics. My only focus is the claim that using an expired car seat is not safe.

"No longer safe to use" is ambiguous - does it mean 100% unsafe or 1% unsafe? But in common language it means that the threshold for the reduction in safety is low enough to recommend the product to be discontinued from use, without necessarily giving exact numbers. For the purposes of this question I'm interested in confirming whether or not expired car seats are more likely to cause injury/death than a new seat by a statistically significant margin.

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    It would be very interesting to know whether an expired seat is safer or less safe than no seat. And whether a good but expired seat is safer or less safe than a cheap, low quality seat.
    – gnasher729
    Commented Dec 12, 2023 at 20:25
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    This is a really good question. Almost all of the attempted answers I can find online approach it from the perspective of how a court would rule on an injury liability lawsuit (against the consumer if the seat was expired), not on what the science is on car seat safety. Commented Dec 14, 2023 at 22:39
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    I note the complaints that this question is making a leap from "Manufacturers don't warrant seats after a certain age" to "Manufacturers are implying seats become dangerous". But this seems to be the same leap made by the second source, and seems like it should be addressable in an answer.
    – Oddthinking
    Commented Dec 25, 2023 at 23:05
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    @Oddthinking yes, and "we know that you’d do anything" implies that we will "spend spend spend" on new seats when we could have been a little more thrifty with an older seat that is technically expired but still in good shape and reasonably safe. I know that the seat manufacturers have no interest in digging up evidence that these older seats might actually be safe, which means it's up to the rest of the world to do this research so that we can make our own informed decisions. Commented Dec 26, 2023 at 11:35
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    Lots of investigation by someone who had the same question as you, and also interpreted the statements of manufacturers and retailers as claims that expired seats are unsafe: marketplace.org/2019/11/14/… Commented Dec 26, 2023 at 22:44

2 Answers 2

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Answers that say "no such research studies have been published" are the hardest to write. I wish I could just say "such research exists, and my proof is the following example", but I will make an effort to justify the claim that research studies as you have described, have not been published in the public domain, but research papers about the science (with no mention of car seats) do exist.

No such research papers found

"Is there any research supporting this statement? I.e. companies buying a bunch of used car seats from Craigslist, putting them into a car crash test and then seeing diminished performance. Or alternatively, independent companies buying brand new seats and comparing them in a car crash test to "expired" 10-year-old seats?"

Despite having about 20 years of experience with using Google Scholar as a professional researcher in materials science and other scientific disciplines (and before the emergence of Google Scholar, having years of experience with ISI Web of Knowledge and other web-based tools for finding research papers), I have not been able to find any research papers that compared new seats to expired ones in crash tests.

Currently I have 28 related tabs open in my browser, each with a different webpage that I found from my Google and Google Scholar searches to find such research papers, and this is after I only selected the ones that I thought were most likely to report such research or at least to cite research papers, and none of them reported or cited any studies resembling what you describe. In a subsequent section I'll provide some of the related research papers that I did examine, but first I'll try to address the point about no research papers being found.

Cost of such research

What I did find about car seat research

Based on searches on Google Scholar and Google, almost all results are "secondary" sources such as from:

Government entities:

Health websites:

News outlets

Informational resources for parents

Car seat companies

None of the above cited one research paper!

Here are the limited research papers or journal articles found:

These are mainly secondary journal articles, or primary research papers about something related tangentially:

In addition to Google Scholar providing an unusually large number of "secondary" sources in the "top" results for all three of my searches (car seat expiry, car seat expiration date, car seat expiration), many of the "scholarly" results that it provided were not journal articles (I found the prevelance of such results here to be especially unusual, indicating further that if any primary research papers like the ones you describe exist, they are not really "accessible"). Here are some of those articles:

  • Diverting Car Seats from the Waste StreamAn Investigation into the Reuse and Recycling of Children’s Car Seats (PDF) (Report commissioned by Zero Waste Washington, produced by CoolMom,and funded through a grant from the Washington State Department of Ecology).

    • Despite this being just a "report" rather than a primary journal article, browsing through it felt like I hit a jack pot because it actually was the closest that I felt that I had come to finding the type of research that you describe, for example it actually has citations in a scholarly format

    • "Car seat expiration dates were developed by manufacturers in part through the testing of materials, which is performed to ensure that the materials can withstand repeated stresses such as sudden stops, a child’s movements, and the temperature extremes within a closed car (24). Additionally, changes to car designs, as with the LATCH safety system for securing car seats, and changes to safety testing standards can make older seats harder to use or no longer considered safe. For instance, the National Highway Traffic Safety Administration (NHTSA) released a proposal in 2014 that would require that car seats designed for use up to 40 lbs.undergo side-impact safety testing, testing which is currently voluntary (25). As older seats “age out” via their 16expiration dates, car seats that can better protect children in aside impact collision would become the new stand"

    • However, disappointingly, reference 24 is literally the article that I already provided above, by the Vancouver Island Car Seat Technicians, and it does not report any primary research nor does it provide a single citation.

What about research about the fundamental science?

Even though my above report indicates that for the type of research study that you described (e.g. doing crash tests with new versus expired car seats), no publicly available article exists, research articles are available about the fundamental science (materials science and matter modeling) lying underneath the claims that car seats expire. Such articles will not show up in Google or Google Scholar searches like the ones I did for the above report, because these articles will not be about car seats, but rather about the science of plastics in general. For example, the fourth result for me in Google Scholar when I searched "plastic deterioration" (after searching "plastic expiration", "plastic shelf-life" and "plastic lifespan" only to find the first few results of each being off-topic) was this article. I can populate this list with more and better articles over time.

When searching for my own answer to send it to a friend, this was the second result on Google when I searched "car seats expire Stack Exchange". The first result was Why do car-seats have an expiration date? (Engineering Stack Exchange) which has answers (that are again not backed up by research or even any references), but may provide insight about the reasons why the expiry dates exist.

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    Thank you! I somewhat suspect that car companies did run such tests but failed to produce a statistically significant outcome so they’ve quietly shelved the data. Commented Dec 18, 2023 at 23:49
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    That's possible! I think it is indeed the car companies that are most likely to be able to afford the crashing of so many cars. It would be much less affordable for entities that have to purchase the cars from those companies. Even for the car companies, it must be very expensive to crash cars and purchase crash test dummies (which can cost $1 million each) in a large enough quantity to get statistically significant results. They might do crash tests on "valid" car seats, but their incentive to spend money on testing "expired" seats might be negligible compared to what's important to them. Commented Dec 19, 2023 at 0:32
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    @JonathanReez Despite the data here showing a lack of research here, I do not think that we should be quick to draw any conclusion on the safety aspect (for or against). These things are made with plastic and other inorganic materials, and they do degrade with time. And when they are kept in a car, they also get very hot and thus face differing thermal stress. So it may very well be that the expiry date is based on research on how long the materials used to make the car seat can survive such different environmental stress. The answer may very well be in material science research.
    – sfxedit
    Commented Dec 19, 2023 at 15:41
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    @sfxedit OP made it "off topic" in their post, but car seats expire for the same reason that drugs do: they haven't been tested past that point. That's it. There's no additional conspiracy required, there is no incentive for a manufacturer to test further and not enough incentive for anyone else to do it. The recommendation to not use things after they are expired is not because of evidence of harm, but because of lack of evidence of safety. None of the sources OP references claim anything else. Commented Dec 19, 2023 at 22:39
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    @sfxedit Food is a bit different: most food has a "best by" date rather than an expiration date. It's less about safety, and more about quality (the manufacturer in particular doesn't want people to assume their product has a bad texture or something because it's been on the shelf too long, even if it's perfectly safe). Commented Dec 19, 2023 at 23:03
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Some are, some aren't

Just like some milk is ok to drink after the expiry date and some isn't.

Car seats are primarily made of plastics and foam and are located in cars that can have extremes of temperatures. Depending on where you live and how or if the car is garaged, temperatures can be above 80°C and below -40°C although usually not both on the same day. They are also exposed to direct sunlight and foams are not UV stable and rely on fabric coverings for protection which is not 100% effective even if intact.

This brings us to the second issue. Car seats are exposed to children. It remains an open question whether children or nuclear bombs are more damaging to materials. Children wriggle, squirm and twist and all this puts strain on the buckles, belts and anchors that the seat depends upon to keep the child safe in an impact. A nick in a belt can be catastrophic in a high-speed impact. This is, of course, equally true of adult seatbelts but I have never seen an adult rhythmically throw themselves forward against the restraint: I've seen plenty of children do it.

Finally, car safety systems, including child seats are constantly being improved. A seat designed 6 or 10 years ago is, all else being equal, less safe than one designed today to fit in today's cars. There are UN recommendations on child restraints which may or may not be reflected in local law or practice. Notwithstanding, seats manufactured before 2015 will not be designed in accordance with UN Regulation No 129 which was issued in that year.

The expiry date represents a consensus, usually industry established and varying by nation (e.g. 6 years in the USA, 10 years in Australia), beyond which problems are increasingly likely to emerge. It is also probably not a coincidence that the timeframes will usually cover a single families use of a child seat.

There will be some car seats that are well-treated and well-maintained that will be safe well beyond the use-by date. Equally, there will be some that are mistreated and abused that may be unsafe as soon as they come out of the box. The use-by date is simply an acknowledgement that things don't last forever and that a relatively low-cost piece of safety-critical equipment used in an environment where everybody knows that there is not going to be regular testing and maintenance should not be relied on indefinitely.

References:

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    There are a lot of claims being made here, and it isn't clear which ones are your opinion and which are substantiated empirically. Can you tied them back to the references given?
    – Oddthinking
    Commented Sep 1, 2021 at 8:42
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    I read the first article, and its 'evidence' boils down to "Largely it’s about common sense" which is an empty statement. (It clearly isn't "common sense" or no-one would need to read the article and no-one would be challenging it.)
    – Oddthinking
    Commented Sep 1, 2021 at 8:43
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    I read the second article. It was reviewed by am MD? But it is just a repeat of the claims in the question. There is no evidence to support it.
    – Oddthinking
    Commented Sep 1, 2021 at 8:45
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    less safe than one designed today to fit in today's cars => needs examples of safety-related changes in car seats over the past 10 years. "Designed for side airbags" might be it but would require showing: what exactly changed in the design, how it affected safety ratings and in which year these changes were introduced. No handwaving, no weasel words. Commented Sep 1, 2021 at 15:53
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    @ChrisHunt greenhouse effect actionnews5.com/2019/06/26/…
    – Rob Watts
    Commented Sep 2, 2021 at 16:23

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