Every person practicing dental hygiene shall at all times display his license in a conspicuous place in his office in plain view of patients. The provisions of this section shall not apply to any dental hygienist while he is serving as a volunteer providing dental hygiene services in an underserved area of the Commonwealth under the auspices of a Virginia charitable corporation granted tax-exempt status under § 501(c)(3) of the Internal Revenue Code and operating as a clinic for the indigent and uninsured that is organized for the delivery of primary health care services.