A system to assess risk of changes to brain white matter includes a sensor system adapted to measure an energy of each impact to an individual belonging to a sub-population that causes a repetitive head impact (RHI). A head impact dose equivalent number (HIDEN) process calculates a HIDEN for a most recent RHI event based on the energy of the most recent RHI event, a time of the most recent RHI, and all previously recorded RHI events available in a non-volatile memory. A sub-population function process calculates a risk assessment number based on the sub-population and the HIDEN. A risk assessment notification means provides a notification of the risk assessment number. A method to assess risk of changes to brain white matter based on a head impact dose equivalent number and a method to define a sub-population function to convert a HIDEN to a risk assessment value is also described.
