Fungal exposure can also come from any volatile compounds
(VOCs) that a fungi/mold creates through primary or secondary
metabolism that then becomes airborne. (Primary metabolic
processes are those necessary to sustain the life of an organism.)
These volatile compounds may be constantly created as the fungus
consumes its food source during the primary metabolic process. VOCs
can irritate the mucous membranes of the eyes and respiratory
system.

Fungi that consume certain organic sources can release highly toxic
gases. For instance, a fungus that grows on wallpaper often releases
toxic gas arsine directly from the wallpaper that contains arsenic
pigments. Thus, fungi and molds can release dangerous materials
when they break down the host material. This can cause mucous
membrane irritation in sensitized individuals.

Fungal volatile compounds may impact the "common chemical sense"
which senses pungency and responds to it. This sense is primarily
associated with the trigeminal nerve. The sensory and motor nerves
respond to pungency by trying to hold the breath, discomfort, or
through sensations such as itching, burning, and skin crawling.
Changes in sensation, swelling of mucous membranes, constriction of
respiratory smooth muscle, or dilation of surface blood vessels may
be part of fight or flight reactions in response to trigeminal nerve
stimulation. Reactions often include a reduced attention
level, general disorientation, lowered reflex time, dizziness, etc.

Volatile Compounds found in or around homes can be responsible for
mucous membrane irritants. It is thought that fungi can add to the
already existing compounds when breaking down certain organic
substances. A mold-contaminated building may have a significant
contribution from its fungal contaminants that is added to common
VOCs---building materials, paints, plastics and cleaners. VOCs in
general can result in symptoms that include lowered attention span,
headaches, lack of concentration, and dizziness.

Reaction to Mold Odors

Some individuals have very strong reactions to the smells given off
by molds. Among humans, there is a high degree of variation in ability
to detect these odors. Certain individuals can detect low levels of
VOCs, while others can only detect relatively high levels. Those
individuals who are particularly susceptible to mold odors may react
with headache, nasal stuffiness, nausea or even vomiting. Asthmatics
often exhibit symptoms when exposed to certain odors.