Research has shown that head hair grows at an average rate of approximately one half inch every month.  In some instances hair may grow slightly slower resulting in a time variance of about one week.  It is important to note that head hair and body hair grow at significantly different rates.  Hair grown below the neck grows much slower than head hair, and therefore body hair is used for drug testing for drug use within the previous year rather than past 90 days and is trusted for their drug testing services.