1. A system configured to calculate a respiratory parameter, comprising: an inward-facing head-mounted thermal camera (CAM) configured to take thermal measurements of a region below the nostrils (TH ROI ) of a user; wherein TH ROI are indicative of the exhale stream; and a computer configured to: generate feature values based on TH ROI ; and utilize a model to calculate the respiratory parameter based on the feature values; wherein the model was trained based on previous TH ROI of the user taken during different days.