Data shows that improving indoor ventilation can reduce the risk of virus transmission, and is recommended by the CDC. In this article, we’ll cover the ASHRAE recommended air changes per hour for offices, homes, schools, residential and hospitals, using the ASHRAE 62.1 and 62.2 guidelines.
ASHRAE Recommended Air Changes Per Hour
In the US, ASHRAE sets the minimum outdoor air ventilation rates for buildings under the ASNI/ASHRAE Standard 62.1 and 62.2 guidelines. These standards specify how much outdoor air should be brought into a room every hour and are based on occupancy and room size.
Read More: What is Air Changes Per Hour (ACH) & How to Calculate
The ASHRAE 62.1 (“Ventilation and Acceptable Indoor Air Quality in Residential Buildings”) recommends homes receive no less than 0.35 air changes per hour of outdoor air to ensure adequate indoor air.
For other spaces like offices, shops, and schools, the ASHRAE 62.1 standard doesn’t give a fixed number. Instead, airflow rates are based on the size of a room, its use (e.g. school, office, sports arena), and the number of people inside. These can be used to calculate exact airflow requirements for a certain space.
The table below provides a summary of the ASHRAE recommended air changes per hour for common building types. These air changes per hour are based on typical room sizes and occupancy rates, including for homes, hotel rooms, offices, schools, and shops.
Learn More: Air Purifiers & COVID-19 Protection for Schools »
The table above gives approximate air changes per hour for schools, homes, hotels, shops, and restaurants. Exact ventilation rates for a given space should be calculated based on the ASHRAE 62.1 standard. But the rules below are helpful starting points for calculating the recommended air changes per hour for your space.
Doubling Room Occupancy Means Doubling Air Changes Per Hour
Ventilation and air change rates are calculated on a per-person basis. If the number of occupants in a room doubles, the required ventilation rate or air change doubles. This rule can be useful for office spaces as the occupancy level changes.
Bigger Spaces Require More Outdoor Air
This rule is simple. Ignoring the number of people in a room, a room that’s 100sqm (1076 sqft) requires twice as much outdoor air as a room that’s 50sqm (538 sqft).
When Should the ASHRAE Standard Not be Used?
The ASHRAE Standard is a useful guideline for ventilation and air change rates in homes, offices hospitals, and classrooms. However, there may be some situations where the recommended ventilation rate here is too low. The ASHRAE Standard outlines two of these situations:
- Areas with smokers. In areas with smokers or environmental tobacco smoke, the required air changes per hour will be higher.
- Areas with sources of harmful emissions. If an area has a high level of harmful emissions such as VOCs, then you may need to increase ventilation further or use an air purifier.
Read more: What are VOCs and how to can we remove them from the air →
ASHRAE Recommendations on Air Changes For Viruses and COVID-19
The ASHRAE guidelines give general ventilation rates to maintain a comfortable indoor environment. They do not cover environments with a high concentration of viruses. That could be hospitals or in the context of COVID-19, offices, schools, or restaurants with infected people.
For areas of higher virus risk, the ANSI/ASHRAE/ASHE Standard 170-2017 or the CDC guidelines should be followed. The ASHRAE 170-2017 states a recommended number of outdoor air changes per hour of 2, with the total air changes required varying from 6-12 (depending on the location in the hospital).
| Hospital Area | Minimum Outdoor ACH | Minimum Total ACH |
| Critical and Intensive Care | 2 | 6 |
| Airborne Infection Isolation Room | 2 | 12 |
| Examination Room | 2 | 6 |
Similarly, the CDC recommends 6-12 air changes per hour for airborne infection isolation rooms (AIIB).
If dealing with viruses such as COVID-19 or other airborne infections, it is therefore recommended to have a higher ventilation rate, in the proximity of 6-12 air changes per hour.
Air Changes With Air Purifiers
Aside from increasing ventilation, an air purifier can also help reduce the risk of viruses indoors. Different from ventilation that brings totally new (but possibly dirty) air into a room, a purifier cleans the existing air. For example, one Blast commercial air purifier can clean the air in a 40 sq m room nearly 9 times in one hour. In other words, the air purifier provides nearly 9 air changes per hour for a 40 sq m room.
Read More: Calculating air changes per hour of an air purifier
Bottom Line:
The ASHRAE recommended ventilation rates for schools, offices, shops, restaurants and homes varies from 0.35 – 8 air changes per hour. When dealing with places that may contain viruses, the recommended air changes per hour are higher, approximately 6-12.
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