Most likely the outside air but another possible answer would be the brounchus, lungs, windpipe, etc.
They are found scattered among the epithelial lining of organs, like the intestinal and respiratory tract. They are also found in the trachea, brounchus. They are also in the respiratory tract, small intestines, the colon, and conjunctiva in the upper eyelid .This information was all found on wikipedia so i cannot take all of the credit for this.