I am reading Oral Medicine: Diagnosis and Treatment by Greenberg and Glick, Chapter 2 - Evaluation Of The Dental Patient: Diagnosis and Medical Risk Assessment.
Under Cranial Nerve Examination, subsection Cranial Nerve I (Olfactory Nerve), the author says:
Olfactory nerve function is traditionally tested by closing one of the patient's nostrils with a finger and asking if the patient can smell a strongly scented volatile substance such as coffee or lemon extract. The test is then repeated for the other nostril. The patient should sniff strongly to draw the volatile molecules well into the nose. This procedure tests for olfactory nerve function only when the nasal airway is patent to the olfactory receptors and when the substance being tested does not produce a response solely on the basis of chemical irritation of nonspecific somatic sensory receptors in the nasal mucosa. Such responses are due to stimulation of branches of the trigeminal nerve.
I tried searching for it on Wikipedia, but it doesn't say anything about nonspecific somatic sensory receptors.