Hearing loss, deafness, hard of hearing, anacusis, or hearing impairment is a partial or total inability to hear. In children it may affect the development of language and can cause work related difficulties for adults. It is caused by many factors, including: genetics, aging, exposure to noise, illness, chemicals and physical trauma. While test results are expressed in decibels, hearing loss is usually described as mild, mild-moderate, moderate, moderately severe, severe, or profound. Hearing loss is usually acquired by a person who at some point in life had no hearing impairment. In some cases such as due to disease, illness, or genetics, it is impossible to reverse or prevent. Hearing aids and implants are partially effective for many. Stem cells have shown promise in the repair of neurologic damage leading to hearing loss.