A crucial factor here is the detection window. For example, hair follicle testing can be able to detect drug use for up to 90 days rather than the 2-3 day range of urine tests. This longer window could be appealing to employers who want a clearer view of potential substance use. But keep in mind that a positive hair test does not necessarily mean ongoing impairment. Social drug use, even months prior, could set off a positive reaction. The second is the invasiveness and cost of each test. Tests of hair follicles are generally more invasive, requiring a hair sample to be pulled from the scalp. Urine tests, however, are less intrusive. Furthermore, hair follicle testing costs more than urine testing. At the end of the day, the choice of test must be determined based on the circumstances of the workplace and the dangers of possible substance use. In safety-sensitive positions where existing impairment is a major risk, urine testing may be more practical given its emphasis on new use. But for jobs where prior drug use would be more pertinent, hair follicle testing could be used, bearing in mind the limitations of the extended window of detection. Note that other approaches (e.g., performance testing) may also be useful. These measures measure a person's capacity to do tasks associated with work, and provide a direct indicator of impairment.