All helmets must comply with the safety standards of your own country. The European standard is CE EN 1078. So first of all carefully check that the helmet has the correct label. Then make sure it fits perfectly to your head (not too tight and not too loose). This means you have to wear it properly (which is not usually possible if the helmet is blistered as it might happen when displayed in supermarkets).

The helmet is not your ‘life insurance policy’ but international researches have pointed out that it has a variable protection effect. Knowing whether helmets reduce the risk of head injuries by 88%, by 65% or by 50% (just to quote some data) is just important for statistics purposes and a secondary factor if we talk about prevention. The most important aspect is that helmets reduce the risk of head injuries!

Yes every time during a fall the helmet has suffered a strong shock, even if it does not look damaged. Each impact damages the material the helmet is made of, and your helmet cannot guarantee the same level of protection any more.