Presets are design values for airborne organisms at 254 nm, shown in m²/J. The calculator works in cm²/µWs, where 1 m²/J = 0.01 cm²/µWs. Values on surfaces are lower, and mold spores are more resistant, so enter a custom k for those.
Required Dose by Inactivation Level
Full Results Table
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How to Use This Calculator
Coil/drain pan applications are continuous, lower-dose surface treatments. In-duct applications need much higher dose delivered in the brief time air passes through the irradiated zone. Upper-room applications use room air mixing over longer exposure periods.
Different microorganisms have very different UV susceptibility, expressed as the rate constant k. The presets are 254 nm design values in air, and they differ by a factor of about 8 between adenovirus and tuberculosis bacteria. Mold spores are generally more resistant than these organisms, so use the custom option with a k from a source you trust.
Choose your target inactivation percentage, commonly 99% or 99.9% for HVAC applications. Enter the UV irradiance from your lamp system's specification sheet or a field measurement with a UV radiometer.
For in-duct air disinfection, air velocity determines exposure time in the irradiated zone. The calculator computes required exposure time and flags whether your system's zone length is realistic given typical duct velocities.
UV-C Germicidal Irradiation in HVAC Systems
UV-C germicidal irradiation has been used in HVAC systems for decades, well before it saw renewed public attention during the pandemic. The physics is well established: UV-C light at germicidal wavelengths damages the DNA and RNA of microorganisms, preventing them from reproducing. What varies enormously between installations is whether the delivered dose actually achieves meaningful inactivation, which depends on getting the sizing math right.
The Dose-Response Relationship
Microbial inactivation by UV-C follows a first-order kinetic model: the fraction of organisms surviving decreases exponentially with dose, according to the organism's UV rate constant, k. The relationship is: survival fraction equals e to the power of negative k times dose. A higher k value means the organism is more UV-susceptible, requiring less dose for the same inactivation percentage. a NIST Journal of Research paper on germicidal UV describes this susceptibility constant (Z or k, in m²/J) as the factor that turns irradiance in W/m² into equivalent air changes per hour, ACHeq = Z × E. For influenza virus in air at 254 nm, the design value from ASHRAE research project 1854 is 0.26 m²/J, which gives a 90% dose of about 8.9 J/m² (0.89 mJ/cm²) and a 99% dose of about 17.7 J/m² (1.77 mJ/cm²). Published coronavirus values vary widely, with an average near 0.034 m²/J, a 99% dose of about 135 J/m². This is why "UV-C kills 99.9% of germs" is a meaningless claim without specifying which organism and at what dose: the dose that inactivates 99.9% of influenza (about 26.6 J/m²) inactivates only about 60% of an average coronavirus.
Coil and Drain Pan UV: Surface Disinfection
UV-C lamps mounted to continuously irradiate the cooling coil and drain pan control microbial growth on these wet surfaces, which are otherwise ideal environments for mold and bacterial growth given constant moisture and organic material from filtered dust. This application requires relatively modest dose since it's continuous. A lamp running 24/7 delivers dose over the entire operating period, not in a single brief pass. Beyond microbial control, coil UV has a secondary benefit: keeping the coil surface clean of biofilm improves heat transfer efficiency over time, since biofilm buildup insulates the coil fins and reduces cooling capacity, and the cooling coil calculator shows what that capacity is worth. Organisms on surfaces are generally less susceptible than airborne ones, and mold spores are more resistant than the presets here, so treat a coil result from an airborne k value as optimistic.
In-Duct Air Disinfection: The Exposure Time Challenge
In-duct UV systems aim to disinfect the air itself as it passes through an irradiated zone, typically in the supply duct. The critical constraint is exposure time: at a typical duct velocity of 2 to 3 m/s, air passing through a 0.5 metre irradiated zone spends only 0.17 to 0.25 seconds under UV exposure. In the calculator's example, a 99% dose for influenza needs about 1,770 µWs/cm², which at 2 m/s takes 0.71 seconds at 2,500 µW/cm², a 1.4 metre zone. At 250 µW/cm² the same dose takes 7.1 seconds, a 14 metre zone. Achieving meaningful dose in that brief window requires much higher irradiance than a coil application running continuously for hours. This is why effective in-duct UV disinfection systems use multiple high-output lamps arranged to maximize irradiance within the available duct length, and why minimal single-lamp retrofit installations often fail to achieve meaningful air disinfection despite technically having "UV-C in the system." The air velocity calculator gives the velocity through your duct cross-section.
Upper-Room UV Systems
Upper-room UV fixtures irradiate only the upper portion of a room, above head height and commonly around 2.1 metres or higher, using shielding to prevent direct UV-C exposure to occupants below. Air disinfection relies on room air mixing: convection currents and mechanical ventilation carry air from the occupied zone up into the irradiated zone and back down over time. This gives much longer effective exposure time than in-duct systems, often measured in minutes rather than fractions of a second, allowing effective disinfection at lower irradiance levels. Upper-room UV has been studied for airborne pathogen control in healthcare settings, including tuberculosis in the Tuberculosis Ultraviolet Shelter Study. The California Department of Public Health's appendix on UV inactivation doses compiles dose data from coronavirus and SARS-CoV-2 studies and shows how susceptibility values translate into equivalent ventilation rates. For occupant safety, the ACGIH exposure limit at 254 nm is 60 J/m² over 8 hours, which is why shielding and aiming matter.
Lamp Output Degradation and Maintenance
UV-C lamp output degrades over the lamp's operating life, and manufacturers commonly specify a reduced output, often around 80% of initial, at the end of rated life for standard low-pressure mercury lamps, so check the datasheet for your lamp. Systems should be sized with this degradation in mind, using end-of-life output rather than initial output as the design basis, or implementing a lamp replacement schedule that keeps output above the minimum design threshold. Dust and dirt accumulation on lamp surfaces also reduces effective output over time, making periodic cleaning part of routine maintenance for coil-mounted systems in particular, where the lamp sits in the same airstream carrying filtered particulate.
Frequently Asked Questions
UV-C dose (fluence) is UV irradiance multiplied by exposure time, giving units of µWs/cm² or mJ/cm² (1 mJ/cm² = 1,000 µWs/cm²). Dose determines the fraction of microorganisms inactivated according to their UV susceptibility constant (k). Different organisms need very different doses for the same inactivation percentage. For influenza virus in air at 254 nm the design k is 0.26 m²/J, so a 99% dose is about 17.7 J/m² (1.77 mJ/cm²), and mold spores generally need much more. Use this calculator to find required dose and exposure time for your target organism and inactivation level.
UV-C in HVAC is most commonly installed at the cooling coil and drain pan (continuous surface disinfection controlling microbial growth and biofilm) or in the supply duct (air disinfection during transit). Coil-mounted UV is lower-dose and continuous. In-duct UV needs much higher dose delivered in the brief transit time, typically under one second at typical duct velocities, requiring higher-output lamp arrays than coil applications.
Effectiveness depends on achieving adequate dose within the limited exposure time in the irradiated duct zone. At typical HVAC velocities, exposure time is often under one second, limiting achievable dose without high-output lamp arrays. Properly sized systems can meaningfully reduce airborne pathogen load, but under-sized retrofit installations with minimal lamp counts provide much less protection. Use this calculator to verify your system's dose is adequate for your target organism and airflow velocity.
UV-C at germicidal wavelengths (around 254 nm) is harmful to skin and eyes with direct exposure. In-duct and coil-mounted systems are installed inside sealed ductwork or air handler cabinets so occupants are never directly exposed. Upper-room UV systems use shielding and careful aiming to keep direct exposure away from occupied breathing zones while disinfecting air circulating through the upper zone. Follow manufacturer safety guidelines and applicable electrical and occupational safety codes for any installation.
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