Cyclists must wear a **safety helmet** properly fastened and adjusted to the head to protect themselves in case of accidents, as bicycles are less visible and more fragile than other vehicles. However, there is an exception to this rule: cyclists are exempt from wearing a helmet in **extreme heat conditions**. This exception is due to the fact that wearing a helmet in these conditions could be detrimental to the cyclist's health, increasing the risk of heat stroke. Therefore, in situations of extreme heat, it is safer for the cyclist not to wear a helmet.