It can be safe to use a used car seat, though only under specific circumstances. If a car seat is very old, it may not conform to modern safety standards or might be worn out. Car seats that have been through accidents may also offer a reduced amount of protection, so they should typically be avoided. Factors such as these make it important to know the history of a used car seat before purchasing or accepting one. A good indicator of whether a used car seat is safe is if it has all of the original parts and does not appear visually worn out, though nothing can be a substitute for firsthand knowledge.
Car seat technology evolves at a fairly consistent pace, and new government regulations are passed from time to time. A used car seat can usually be considered out of date when compared to modern units once it is about five or six years old. Units older than that may still be safe, especially if they were gently used and well taken care of, though they often do not provide the maximum amount of safety and protection available.
An old car seat that was previously used by a sibling only a few years prior can almost always be considered safe. In this case, the history of the car seat is well known and it is probably not old enough to be considered outdated. If a friend or relative offers a car seat of a similar age that has never been in an accident or mistreated, that is typically safe as well. Other situations where the history of the seat is not well known may not necessarily be dangerous, though there will be more of a chance that there could be a hidden defect.
Purchasing a used car seat is typically more risky than using one that has a well known history. Before buying a used car seat, it is a good idea to determine whether safety testing was done on the particular model. Consumer protection organizations often do such testing, and the results may be useful in determining whether a car seat that is a few years old is a safe model. If a seat received poor marks a few years prior, it will almost definitely be outdated by current standards. A visual inspection can also be useful, since a car seat that is worn or missing pieces may not be safe.