Ventilation Rates to Design for Airborne Transmission: ASHRAE 241 vs. EN 16798-1 Revision Proposal

Ventilation design methods for airborne transmission are developing and may change ventilation system and air distribution design in the future. In this article the first available method in ASHRAE 241 is benchmarked with new EN Health proposal for the revision of EN 16798-1. Capability to account for the occupant density and unrealistically high ventilation rates have been issues in first methods but developments allow to go closer to today’s Category II ventilation rates if occupancy reductions are carefully considered.

Keywords: target ventilation rate, ventilation effectiveness, infectious respiratory particles, pathogens, viruses, quanta emission, infectious aerosols

While the importance of ventilation was well recognised during the pandemic it still is a question how to design ventilation systems to reduce airborne transmission of infectious aerosols. Main issues are selection of proper ventilation rates and ensuring ventilation effectiveness through air distribution design. General ventilation mostly affects long range transmission meaning that close contact should be avoided during epidemic periods when additional personal protection measures may also be used. In the following ventilation rates designed to limit airborne transmission with ASHRAE 241 [1] and proposal for EN 16798-1:2019 [2] revision are compared in ventilation design.

Most used existing ventilation design criteria in non-residential spaces is based on perceived air quality (PAQ). This represents an outdoor airflow rate diluting respiratory effluents, body odours and material emissions from building to the level that occupants feel acceptable. EN 16798-1:2019 provides ventilation rates that are sufficient for the visitors (unadapted persons) in non-residential buildings and occupants (adapted persons) in residential buildings. Target ventilation rate, in this case outdoor air if no gas phase air cleaning is used, is calculated from number of persons and floor area:

qtot= nqp + ARqB

where

qtot      total ventilation rate for the breathing zone, L/s

n          design value for the number of the persons in the room

AR        room floor area, m²

qp        ventilation rate for occupancy per person, 7 L/(s person) in Category II

qB        ventilation rate for emissions from building, 0.7 L/(s m²) in Category II with low polluting materials and 0.35 L/(s m²) with very low polluting building materials

Design for airborne transmission is based on virus emission of an infected person. In this case the emission source, an airborne pathogen has different source distribution because typically only one infector is expected to be in a room in design conditions. This represents a point source contrary to PAQ when all occupants and surfaces are emission sources representing a distributed source. It is evident that dilution and effective removal of a point source with unknown location in the room is more challenging for ventilation system compared to distributed source.

Available design methods for airborne transmission keep the likelihood of infecting others on the reasonable level to avoid the rapid spread of epidemic. Infection risk is considerably reduced but not eliminated as it is typically aimed that one infector will cause no more than one new disease case corresponding to basic reproduction number R0 = 1. Such methods are relevant for shared indoor spaces (mostly non-residential), but not for health care where more stringent criteria shall be used.

EN 16798-1 new method EN Health for airborne transmission

New method proposed in the revision of EN 16798-1 is intended to be used in combination with one of existing methods (PAQ, specific pollutant or fixed ventilation rates) so that the highest value of the ventilation rate will be used as the design ventilation rate. It is Wells-Riley quanta method for long range transmission with one infector in the room [3]. The risk model applied calculates the ventilation rate reducing the expected number of secondary infections from an infected person to 1.0 over the sequence of interactions the infected person has with susceptible persons during the whole pre-symptomatic infectious period [4]. Wells-Riley method is sensitive to selected quanta values making absolute risk estimation uncertain, but the method provides the same number of new disease cases for all spaces and occupant densities.

Target ventilation (non-infectious) rate for infection risk control = outdoor air + particle filtered air + disinfected air is calculated from number of person and room volume with tabulated values for virus specific parameters given in Table 1:

Q = qq (N − 1) − qr V

where

Q         target ventilation rate for the breathing zone, L/s

qq        quanta emission specific ventilation rate for occupancy per person, L/(s person)

qr        removal rate of virus decay, deposition, air filtration and disinfection, L/(s m³)

N         design value for the number of persons in the room with the distancing >1.0 m to avoid close proximity

V          room volume, m³

Table 1. Tabulated values for virus specific parameters qq and qr.

The method allows to take into account room air cleaners, marked with filtration removal rate kf(1/h) in Table 1. In the case of no air cleaner, kf = 0. Target ventilation rate equation may also be applied to calculate allowed occupancy at given ventilation rate.

To calculate the ventilation rate supplied by air distribution system, the target ventilation rate shall be divided by the contaminant removal effectiveness. In mixing ventilation with distributed source, a default value is 1.0, but in the case of airborne transmission the values tend to be lower with typical range of 0.8 – 0.9 that will increase the ventilation rate because of lower ventilation effectiveness in the case of the point source [5].

ASHRAE 241 for airborne transmission

ASHRAE 241 uses also Wells-Riley quanta method but has different risk model. The risk model applied considers infection risk to a group of people in spaces with fixed size and occupancy, accounting for the probability of an infector being present in a space based on the community incidence. The model operates with the individual risk < 0.1% after one hour of occupancy for 96% of the time, considers a probability of the presence of infected and susceptible people and uses the community infection rate of 1%. Therefore, epidemic spread is not considered by this risk model, but it is a probabilistic model that considers variations in quanta and breathing parameters in input parameter distributions. Ventilation rates (equivalent clean airflow rates per person) providing the risk of 0.1% are calculated for selected spaces with fixed size and default occupant density thus the model strictly applies only for this number of occupants and room volumes that were used. For instance, the modelled classroom has 120 m² and 30 students and the office 1000 m² and 50 persons. The impact of changed occupant density cannot be calculated, as just 20 L/s per person in classrooms and 15 L/s per person in offices is recommended.

Calculation example

Target ventilation rates for model classroom and open office shown in Table 2 are calculated with varying occupant density both with airborne transmission and PAQ calculation methods:

·         EN Health, new proposed method for airborne transmission to EN 16798-1

·         ASHRAE 241:2023 airborne transmission

·         EN 16798-1:2019, Category II, very low polluting building, PAQ

·         ASHRAE 62.1:2022 [6], PAQ

Table 2. Model rooms used in calculation example

Open office results in Figure 1 illustrate the difference of fixed value of ASHRAE 241 and EN Health depending on occupant density. At typical occupant densities close to 10 m² the results are similar. The tendency of EN Health is different from PAQ methods as the highest ventilation rate per person is achieved at the highest occupant density. EN Healh is intended to be used in combination with EN-16798-1 PAQ that provides higher ventilation rate at low occupant densities (solid line in the figure). ASHRAE 62.1 that operates with adapted persons provides the lowest ventilation rate.

Figure 1. Target ventilation rates in model open office. Dashed lines represent the values that should not be used, as the higher value provided by EN methods is to be used.

In the model classroom, ASHRAE 241 provides excessively high ventilation rate resulting in as high air change rate as 11 1/h. EN Health is close to EN 16798-1 PAQ at common occupant densities. High quanta emission at speaking may explain why ASHRAE 241 is an outlier. EN Health assumes infected student speaking 5% of the time, but considerably higher value has been used in ASHRAE 241.

Figure 2. Target ventilation rates in model classroom.

Conclusions

Ventilation design methods for airborne transmission are developing. The first available one, ASHRAE 241 is not capable to account for the occupant density and provides unrealistically high ventilation rates in classrooms. EN Health proposed to be included in the revision of EN 16798-1 provides moderately higher ventilation rates than existing Category II PAQ method only at high occupant densities. At lower occupant densities, existing Category II ventilation rates are sufficient. Independently of which airborne transmission design method is used, the target ventilation rate shall be divided by contaminant removal effectiveness for which some default values will be recommended in the standard to consider lower ventilation effectiveness in the case of the point source.

References

[1]     ASHRAE Standard 241:2023, Control of Infectious Aerosols.

[2]     EN 16798-1:2019 Part 1: Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics.

[3]     Amar Aganovic, Guangyu Cao, Jarek Kurnitski, Pawel Wargocki, New dose-response model and SARS-CoV-2 quanta emission rates for calculating the long-range airborne infection risk, Building and Environment, 2022 https://doi.org/10.1016/j.buildenv.2022.109924.

[4]     Kurnitski, Martin Kiil, Alo Mikola, Karl-Villem Võsa, Amar Aganovic, Peter Schild, Olli Seppänen, Post-COVID ventilation design: Infection risk-based target ventilation rates and point source ventilation effectiveness, Energy and Buildings, 2023 https://doi.org/10.1016/j.enbuild.2023.113386.

[5]     Martin Kiil, Alo Mikola, Karl-Villem Võsa, Raimo Simson, Jarek Kurnitski, Ventilation effectiveness and incomplete mixing in air distribution design for airborne transmission, Building and Environment, 2025, https://doi.org/10.1016/j.buildenv.2024.112207.

[6]     ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality.