Objective The aim of the study is to evaluate the metabolic demands and internal breathing environments when covering an N95 with a surgical mask, cloth mask, and/or FS. Methods Three N95 models approved by the National Institute for Occupational Safety and Health were evaluated under six covering conditions using the National Institute for Occupational Safety and Health Automated Breathing and Metabolic Simulator. All conditions used one trial with each N95 for six incremental 5-minute work rates. Inhaled oxygen and carbon dioxide concentrations, peak inhaled and exhaled pressures, and inhaled wet-bulb and dry-bulb temperatures were measured continuously and averaged across all work rates and covering conditions. Conclusions Results suggest that metabolic demands and internal breathing environments are significantly impacted by all combinations of coverings tested when compared to N95 only.