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What does 'Air Changes Per Hour' (ACH) Mean?

Air changes per hour (ACH) is a useful measurement to understand if you’re looking for better indoor air quality in your workplace.

  • An air change is when the entire volume of air in a room is replaced with ‘new’ or recirculated air.
  • The number of times this whole volume of air is replaced each hour is the number of air changes per hour.

When you understand, and know how to measure, air changes per hour, you’ll be able to determine:

  • The amount of ventilation needed in every space in your workplace
  • How well ventilated all your current spaces currently are
  • If, and where, you need to improve your workplace air quality

Why are air changes per hour important?

The entire volume of air in every room, or space, needs to be filtered so that all the pollutants and virus particles can be removed.

This air change should happen several times every hour, depending on the use of the room, number of occupants and other factors.

Once you know the ACH you can help to protect the health and wellbeing of everyone using that space, by checking whether you already have the right air purification or ventilation in place and knowing which steps to take if not.

For example, the ACH requirements of a room can help you work out exactly which type of air purifiers, and how many, you need for that space.

Why does air quality matter?

Indoor air quality and ventilation affects many different aspects of the workplace.

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Health Risk

Eliminate airborne pollutants, TVOCs and virus particles less than 2.5 microns that pose the greatest health risk

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Odours

Remove odours that can otherwise distract and disturb occupants, impacting on perceptions of your facility

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Room Temperature

Maintain a comfortable temperature to prevent the most common office complaint

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Humidity Levels

High levels of humidity are ideal conditions for mould and bacterial growth.

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Higher Productivity

Create the optimum environment to benefit from a highly energised and productive workforce

Most employers are all too aware that indoor air quality is an important measure of workplace satisfaction, with a Fellowes survey finding that 73% of people agreed that the quality of the air they breathe directly impacts their health and well-being.

ACH can also help you find the right balance between energy use and adequate ventilation.

CIBSE (The Chartered Institution of Building Services Engineers) advises building owners and operators to reduce energy use while providing workplaces and public spaces that are appropriately ventilated for the activities taking place in them.

How to measure air changes per hour/ACH

To calculate ACH you need to take four steps:

1. Find out the cubic feet per minute (CFM) of air entering the room.

  • If a ventilation or air purification system is already in place, you can check the unit for this information.

2. Calculate the volume of air flow in cubic feet per hour by multiplying the CFM by 60.

  • Volume of Air Flow = CFM x 60.

3. Work out the volume of air in the room by multiplying the length, width and height of the room, in feet.

  • Air volume = H x W x L (in feet).
  • A room 10 feet high x 50 feet wide x 100 feet long, would have a total air volume of 50,000 cubic feet.

4. To find the ACH, you divide the cubic feet per hour (step 2) by the volume of air in that room (step 3).

  • Air changes per hour (ACH) = (CFM x 60) / air volume in cubic feet.

Recommended air changes per hour in a workplace

This gives you an idea of typical recommended air changes per hour across different workplace environments:

Environment
Recommended ACH
Office
6-8
Conference room
8-12
Break room
7-8
Food service/Kitchen
14-18
Public toilet
10-12
Classroom
6

Air changes per hour and HVAC systems

If you’re wondering whether your HVAC (heating, ventilation and air conditioning) system is changing the air enough, the simple answer is probably not.

HVAC systems achieve between one and three air changes per hour, and even less in older systems. As you can see from the recommendations above, this number of air changes is not enough for meeting air quality standards.

Added to the fact that HVAC systems do not filter the air to an adequate standard, because their filters cannot remove all particulates, it’s unlikely that any mechanical ventilation system will be able to deliver the air quality, or number of air changes, that you need.

Read our article "Are workplace ventilation systems failing to clean the air we breathe?" to learn more about HVAC systems and indoor air quality.

How do I improve our workplace indoor air quality?

Ultimately, you’re likely to need an air purification management system to complement your existing HVAC system.

First though, you need to get an understanding of the health of your workplace air through indoor air quality monitoring. Once you understand the indoor air quality in your environment you can then take the appropriate steps to improve the air you breathe.

Discover how to monitor indoor air quality, and deliver a healthier, more comfortable workplace for you and your colleagues by clicking on the button bellow.

Air Quality Monitoring

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