Air change rates in NHS general wards: weighing airborne infection risk against the energy, cost and carbon of ventilation

Health Technical Memorandum 03-01 sets a design rate of six air changes per hour (ACH) for general wards in England, but the infection-control benefit and the operating cost of that rate are rarely set side by side. This study combines a Wells-Riley airborne infection model with an annual energy, cost and carbon model for a four-bed ward bay of 172.8 m³, using current UK energy prices and 2026 carbon factors. For a single infectious patient over 24 hours, modelled infection probability for each susceptible patient fell from 8.3% to 5.2% for tuberculosis and from 6.2% to 3.9% for SARS-CoV-2 when ventilation rose from 6 to 10 ACH. Each additional air change cost about £263 and 318 kgCO2e a year with a run-around coil and gas heating, and the benefit of each extra air change fell steadily as the rate rose. Reaching 10 equivalent air changes with a portable HEPA unit instead of extra outdoor air cut the added running cost by 80–89% and the added emissions by 90–95%. If an existing air handling unit is simply run faster, the added cost rises to about £4,700 a year because fan power grows with the cube of airflow. The results support keeping the HTM baseline for outdoor air and using air cleaning to go beyond it.