Measurements were performed simultaneously with two APS instruments (TSI 3321) and one Optical Particle Sizer (OPS, 0.3 -10 μm, TSI 3330) sampling from the same custom-printed funnel. A comparison of measurements between the two APS instruments was linear, with a slope that deviated from 1 owing to different sensitivities of the instruments (Extended Data Fig. ). The OPS detected significantly more particles than the APS (up to a factor of 2), a consequence of the lower size detection limit of the OPS (to 300 nm) compared to the APS (to 500 nm) (Extended Data Fig. ). Including these smaller particles in our analysis significantly increases the number concentration but does not significantly change the particulate mass concentrations from the expiratory activities.