How much does an impact reduce helmet's ability to protect?

Question

I read the answers to "When or how often should I replace my bike helmet?", but unfortunately none of the answers there cites any references.

Have there been any studies quantifying the claim of reduced protection after single impact, e.g. in terms of increased risk of injury?

It's easy to see reasons why manufacturers would want you to believe any impact kills the helmet - it's hard to set limits on potential damage, and buying a new helmet means additional profit.

Horse riding helmets come with similar warnings, but my first one seemed to survive several years while I was learning show jumping - including way more falls than I could have afforded new helmets.

Answer 1

There is a lot of good information about this here. One paragraph from the article outlines the basic idea of this:

Crushing the cell walls destroys the impact management ability for most stiff foams, so the helmet has to be replaced after a single impact. The crushing is not always visible and can be hidden by the outer shell. The foam can also recover some of its thickness over a period of hours, but not its ability to manage impact. Crushed and partially recovered one-use foam will feel rubbery and soft. Experts measure the foam thickness carefully for crush, but for consumers the recommendation has to be "replace after every impact." Often the damage is not apparent in a casual inspection, even when there are cracks in the foam. And the rider often underestimates the impact because crushing of the foam dampened the shock.

It goes on to explain the different types of technologies (with some links to scientific information) used in bike helmets. While your helmet survived all the falls, it was most likely offering you less and less protection from the falls.

Answer 2

Your dressage helmet had 2 advantages, in one sense, which is that you were landing on a prepared dirt floor in most instances, and that they usually included a harder exterior shell that helps protect the protection foam.

That said, there is no way to know if it was still doing its job after the first head to ground impact.

Even if it still looked good, and you still wore it for years, its impact protection ability drops each time the foam is compressed in the helmet.

The bicycle helmet has nothing but this foam, so damage is more immediate and visible, but your riding helmet was damaged, too.

The impact that damages a helmet is as often from the momentum of the riders head as it is the impact with ground, so a dressage helmet will usually have the same kind of damage. It just doesn't show as easily.