Update: CDC warns of wearing mask past 1-hour.

Hypoventilation is a primary cause of significant discomfort while wearing an N95 FFR (Williams 2010). However, studies done by Roberge et al. (2010) indicated that this hypoventilation did not pose a significant risk to healthcare workers over the course of less than one hour of continuous N95 use. When HCWs are working longer hours without a break while continuously wearing an N95 FFR, blood CO2 levels may increase past the 1-hour mark, which could have a significant physiological effect on the wearer (Lim et al., 2006)*. Some of the known physiological effects of increased concentrations of CO2 include:

Increased pressure inside the skull;
Nervous system changes (e.g., increased pain threshold, reduction in cognition – altered judgement, decreased situational awareness, difficulty coordinating sensory or cognitive, abilities and motor activity, decreased visual acuity, widespread activation of the sympathetic nervous system that can oppose the direct effects of CO2 on the heart and blood vessels);
Increased breathing frequency;
Increased “work of breathing”, which is result of breathing through a filter medium;
Cardiovascular effects (e.g., diminished cardiac contractility, vasodilation of peripheral blood vessels);
Reduced tolerance to lighter workloads.