Filtering face piece respirators are the lightest weight and most comfortable respirators. During inhalation, air is drawn through the entire surface area of the face piece— with less resistance and less breathing work compared to half and full face respirators. Despite their light weight, filtering face pieces still have physical stressors that impact the wearer. Inspiratory resistance, expiratory resistance, increased dead air space, and heat stress lead to an increase in respiratory rate, heart rate, and both systolic and diastolic blood pressure when compared to non-respirator wear. These effects are more notable when filtering face pieces are worn during heavy work. Filtering face pieces are described as oil-proof, resistant, or not resistant to oil: P, R, or N, as well as by their percent efficiency at removing airborne particles: 95, 99, or 100.
Reference:
Navy and Marine Corps Public Health Center