• There is currently no evidence of human infection with SARS-CoV-2 caused by infectious aerosols distributed through the ventilation system air ducts. The risk is rated as very low.
• Well-maintained HVAC systems, including air-conditioning units, securely filter large droplets containing SARS-CoV-2. COVID-19 aerosols (small droplets and droplet nuclei) can spread through HVAC systems within a building or vehicle and stand-alone air-conditioning units if the air is recirculated.
• The airflow generated by air-conditioning units may facilitate the spread of droplets excreted by infected people longer distances within indoor spaces.
• HVAC systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing the recirculation of air, and increasing the use of outdoor air. — Julie
Is the important difference between ventilation (used air is deposited outside and new air comes into the system) versus re circulation (used air is reused)? — Peter Beaver