Avoid recirculated air within business buildings and inside automobiles under these conditions.
Most of the time, automobile air recirculation is off. Exceptions are when you are driving behind diesel vehicle or traveling near cow or pig farms. If enabled you may notice louder fan noise as your exhalation is recirculated throughout the cabin. Disable to pull fresh air from outdoors.
If you are traveling by yourself or with those in your household, you are used to breathing in each other's air. If you are a Lyft or Uber driver, you might take your chances with diesel emissions while wearing a mask. Some rideshare drivers have a safety partition for additional protection.
Office buildings with HVAC should similarly avoid recirculating air. COVID-19 (alpha) remains active up to 3 hours in indoor air and 2–3 days on room surfaces at common indoor conditions. Such small virus particles linger airborne and travel long distances carried by airflows in the rooms or in the air extraction ducts of ventilation systems.
Sometimes air handling units and recirculation sections have return air filters. But most of these filters, even HEPA filters may not filter out virus size particles effectively. Whether you are in an automobile, home, or business, pay attention to filters for recirculated air or vent air from outdoors.