Are expired car seats no longer safe to use?

Question

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. Or alternatively, independent companies buying brand new seats and comparing them in a car crash test to "expired" 10-year-old seats?

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.

Answer 1

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:

• https://www.carsguide.com.au/car-advice/car-seat-expiry-date-australia-how-long-do-car-seats-last-80311
• https://www.healthline.com/health/baby/car-seat-expiration
• https://www.washingtonpost.com/cars/yes-car-seats-expire-and-heres-why/2017/02/23/608ce07c-fa05-11e6-aa1e-5f735ee31334_story.html