The dose is influenced by relatively low penetration of beta emissions through materials. The cornified keratine layer of epidermis has enough stopping power to absorb beta radiation with energies lower than 70 keV. Further protection is provided by clothing, especially shoes. The dose is further reduced by limited retention of radioactive particles on skin; a 1 millimeter particle is typically released in 2 hours, while a 50 micrometer particle usually does not adhere for more than 7 hours. Beta emissions are also severely attenuated by air; their range generally does not exceed and intensity rapidly diminishes with distance.