I stumbled across this international study regarding community effects from a large scale NH3 release in 2022. The study wanted to validate or in-validate community complaints years after the event. Residents, numbers unknown, had been complaining of “loss of smell” after being exposed to hundreds of ppm’s of NH3. The study was interesting in that it claimed that some people were incapable of smelling NH3 at 300 ppm (IDLH), but yet could sense their exposure via Trigeminal Sparing. I find this very hard to believe, but the study syas what it says.
When discussing ammonia and olfactory deficits, it is critical to distinguish between temporary sensory adaptation (olfactory fatigue) and permanent physiological damage (anosmia or hyposmia). Ammonia is unique because it stimulates both the olfactory nerve (Cranial Nerve I) and the trigeminal nerve (Cranial Nerve V), which provides the “sting” or burning sensation.
1. Olfactory Fatigue (Sensory Adaptation)
This is the most common deficit and the most dangerous from a process safety perspective.
- Mechanism: Continuous exposure to low-level ammonia (e.g., in a refrigeration engine room) desensitizes olfactory receptors. The brain begins to “filter out” the constant signal to avoid overstimulation.
- Safety Risk: Because the odor threshold for ammonia is low (5 ppm to 15 ppm), it is often considered a “self-warning” gas. However, fatigue can render a person “nose-blind” to increasing concentrations, leading them to stay in an environment that has reached hazardous levels (300 ppm is IDLH).
2. Physical Deficits: Anosmia and Hyposmia
Prolonged or high-concentration exposure can lead to lasting deficits:
- Hyposmia (Reduced Smell): Chronic exposure can cause inflammation of the nasal mucosa, leading to a diminished ability to detect any odors, not just ammonia.
- Anosmia (Total Loss of Smell): While rare from low-level exposure, acute overexposure to high concentrations can cause corrosive “liquefaction necrosis” of the olfactory epithelium. Because ammonia reacts with moisture to form ammonium hydroxide (a strong base), it can destroy the delicate cilia and mucosal barrier required for odor detection.
- Trigeminal Sparing: Interestingly, in many cases of organic anosmia, the patient can still “feel” ammonia due to the trigeminal nerve stimulation (the burning sensation), even if they cannot “smell” its distinct pungent aroma.