# Office CO2 and Fresh Air: How ASHRAE 62.1 Sets Ventilation

> An office's fresh air requirement is the outdoor air supplied to dilute what people and the building release. ASHRAE 62.1's ventilation rate procedure sets it from two parts, a rate per person and a rate per unit floor area, applied to the real occupancy. CO2 shows whether that air is reaching people; it is an indicator, not a pollutant limit, and air changes per hour do not measure it.

## Drowsy afternoons are usually a ventilation problem

Ask people on a crowded office floor how they feel at four in the afternoon and a
familiar set of complaints comes back: heavy heads, stale air, meeting rooms
nobody wants to sit in for long. The usual response is to turn the thermostat
down. The room gets colder and nothing improves.

That is because the complaint is rarely about temperature. **Cooling removes heat;
ventilation replaces the air people have breathed.** Most office air conditioning
recirculates the same air through the cooling coil and back into the room. The
outdoor air that dilutes what occupants exhale is a separate, deliberately sized
quantity, and it is the one most often undersized, poorly distributed or turned
down to save energy.

For a business, the cost is not the air conditioning bill. It is a floor that
people tolerate rather than use, meeting rooms avoided after lunch, and a
facilities team chasing a temperature complaint that is really a ventilation one.

## How ASHRAE 62.1 sets outdoor air for an office

**ASHRAE 62.1**, Ventilation and Acceptable Indoor Air Quality, is the standard
most commonly referenced for office ventilation design in India, alongside the
air conditioning and ventilation provisions of **NBC 2016, Part 8**. Its
ventilation rate procedure builds the outdoor air requirement for a zone from two
parts:

- **A people component**: a rate per person multiplied by the number of people
  the zone is designed for. It covers what occupants themselves release, such as
  exhaled breath and body odour.
- **An area component**: a rate per unit floor area multiplied by the zone's
  area. It covers what the building releases regardless of headcount, such as
  emissions from finishes, furniture and equipment.

The sum is then adjusted for how effectively the air distribution actually
delivers outdoor air to the breathing zone, and, where one system serves several
zones, for how efficiently it shares that air between them. The rates for each
type of space are tabulated in the standard and revised between editions, so the
design engineer sets the values from the edition adopted in the project's design
basis. This page explains the method; it does not give the numbers.

The commercial consequence sits in the people component. **Because it scales
with headcount, densifying a floor raises the requirement.** A system sized for
one seating density will under-ventilate the same floor after it has been
re-planned with more desks, and a small meeting room that seats many people for
an hour needs far more outdoor air per square metre than the open plan beside it.

| Way of expressing ventilation | What it actually measures | Where it helps | Why it misleads for an office |
| --- | --- | --- | --- |
| Outdoor air per person plus per area (ASHRAE 62.1 method) | Outdoor air delivered against the people and building sources it must dilute | Setting the design quantity for each zone | Only valid if the occupancy assumed is the occupancy built |
| Air changes per hour | Total air moved relative to room volume, mostly recirculated | Rooms where the requirement relates to the room itself | Does not say how much of the air is outdoor air, or scale with people |
| Indoor CO2 reading | How well exhaled air is being diluted, in practice | Checking performance, controlling ventilation by occupancy | Ignores pollutants that do not come from people |
| Cooling capacity | How much heat the plant can remove | Sizing the cooling plant | Says nothing about ventilation at all |

## CO2 is an indicator, not a pollutant limit

People exhale carbon dioxide. In a space where people are the main source, the
gap between indoor and outdoor CO2 widens when outdoor air per person falls. That
makes CO2 a practical, cheap proxy for one question: **is the outdoor air the
design intended actually reaching the people in this room?**

It is not a measure of overall air quality. ASHRAE 62.1 sets ventilation as
outdoor airflow, not as an indoor CO2 concentration, and a low CO2 reading proves
nothing about emissions from new finishes, cleaning products, particulates or
polluted outdoor air drawn into the intake. Single-number CO2 thresholds that
circulate online are rules of thumb, not requirements of the standard.

Used properly, CO2 monitoring is valuable:

- Compare indoor readings with the outdoor level, which varies by location and
  time of day, rather than reading an indoor figure in isolation.
- Watch the trend through a day. A room whose CO2 climbs steadily as it fills is
  telling you outdoor air per person is short.
- Place sensors in the breathing zone, away from supply terminals, doors and
  people's faces, and keep them calibrated.
- In meeting rooms and other variable-occupancy spaces, CO2 can drive
  **demand-controlled ventilation**, varying outdoor air with occupancy. ASHRAE
  62.1 sets conditions on its use, and the design engineer decides where it fits.

Sensors are most useful when connected to the controls rather than displayed on
a wall; [building management systems](/services/mep/building-management-systems)
are where that trend data is logged and acted on.

## Why ACH is the wrong way to specify an office

Specifications still arrive asking for "so many air changes per hour" for an
office. The figure can be met while the occupants still lack outdoor air, because
air changes count total air movement against room volume, and in an office that
air is mostly recirculated. It also does not move when headcount does, and it
changes with ceiling height for the same airflow.
[Air changes per hour](/glossary/air-changes-per-hour) explains where ACH is the
right metric. For an office, specify the outdoor air basis and the occupancy it
is designed for.

## Fresh air in landlord-supplied systems

In many multi-tenant buildings, outdoor air is treated centrally and delivered
through a riser at a fixed quantity per floor, sized on the landlord's assumed
occupancy. The tenant's fit-out distributes it. That arrangement works until the
tenant's seating plan is denser than the landlord assumed, at which point the
floor is under-ventilated by design, not by fault.

Questions to settle in design, in writing, with the landlord:

- What outdoor air quantity is allocated to this floor, and on what occupancy?
- Is it cooled, dehumidified and filtered before it arrives?
- Does it run through all occupied hours, including late working and weekends?
- Can the tenant add or increase outdoor air if the layout needs it, and how?

How fresh air fits into the wider plant is covered in
[commercial HVAC systems](/blog/commercial-hvac-systems). Distribution, zoning and
return-air problems that also cause stuffy rooms are covered in
[office HVAC problems after handover](/blog/office-hvac-complaints-after-handover).

## The energy trade-off

In most Indian climates outdoor air is hot, humid or both for much of the year,
and treating it is a significant part of the cooling load. That creates a quiet
temptation to reduce it, by undersizing, by closing dampers, or by switching
fresh air off outside peak hours.

The better answer is to make the trade-off explicit. The design can size outdoor
air to the real occupancy rather than a blanket figure, use demand-controlled
ventilation where occupancy varies, consider recovering energy from exhaust air
where the engineer finds it worthwhile, and control the system so it runs when
people are present. **ECBC 2017** sets energy efficiency provisions for comfort
systems and controls in buildings within its scope; how those provisions apply to
a project, and how they interact with the ventilation design, is for the MEP
engineer to establish.

## Common mistakes

- **Treating stuffiness as a temperature problem** and lowering the set point.
- **Specifying an office on air changes per hour** instead of outdoor air for the
  occupancy.
- **Densifying the floor without re-checking ventilation**, because only the
  furniture changed.
- **Ventilating meeting rooms at open-plan density**, then wondering why they
  feel airless within an hour.
- **Reading CO2 as a full air quality score**, or reading it without the outdoor
  comparison.
- **Closing fresh air dampers to save energy** without anyone recording the
  decision or its effect.
- **Assuming the landlord's fresh air suits your headcount** without asking what
  it was sized for.

## What to ask your designer

- Which edition of ASHRAE 62.1, or which other basis, does the ventilation design
  use, and what occupancy does each zone assume?
- How do meeting rooms and training rooms get their outdoor air?
- Where does outdoor air come from, and is it treated and filtered before supply?
- Will CO2 be monitored, where, and will it control anything?
- How will outdoor air quantities be measured and recorded at commissioning?
- What happens to ventilation if we add desks later?

## Standards referenced

Ventilation and acceptable indoor air quality, including the ventilation rate
procedure, in **ASHRAE 62.1**; air conditioning, heating and mechanical
ventilation provisions in **NBC 2016, Part 8**; energy efficiency of comfort
systems and controls in **ECBC 2017**. Ventilation rates, the governing standard
edition, occupancy assumptions and control strategy for a specific building must
be established by the project's MEP engineer and consultants; nothing on this page
replaces that design.

## Frequently asked questions

### Why does the office feel stuffy even though the air conditioning is cold?

Because cooling and ventilation are different jobs. Most office air conditioning recirculates the same air and cools it; the outdoor air that replaces what people have breathed is a separate, smaller quantity. If that quantity is undersized for the headcount, not reaching the floor or turned down to save energy, the room stays cool and still feels stale.

### What CO2 level should an office have?

ASHRAE 62.1 sets ventilation as outdoor airflow, not as an indoor CO2 limit, so there is no single figure in it to design to. Numbers quoted online are rules of thumb. A CO2 reading is most useful compared with the outdoor level and tracked over a day: if it climbs steadily as a room fills, outdoor air per person is falling short. The design engineer should interpret it.

### Does a low CO2 reading mean the air is healthy?

No. CO2 tracks how well the air people exhale is being diluted. It says nothing about emissions from finishes and furniture, cleaning products, particulates or outdoor pollution drawn in through the intake. That is one reason ASHRAE 62.1 includes an area-based component in addition to the per-person one, and why filtration is a separate design question.

### Can we just add more fresh air to fix the problem?

Sometimes, but outdoor air in most Indian climates has to be cooled and dehumidified before it is supplied, so more of it means more cooling load and energy. The right fix may be rebalancing distribution, adding demand-controlled ventilation to meeting rooms, or increasing the supply to match the real occupancy. The MEP engineer should diagnose before anything is changed.

### Is fresh air the landlord's responsibility or the tenant's?

It depends on the building and the lease. In many multi-tenant buildings outdoor air comes from a central treated fresh air system at a fixed quantity per floor, and the tenant distributes it. The handover schedule should state the quantity allocated, the occupancy it assumed and who operates the system. If your headcount is higher, the gap has to be resolved with the landlord in design.

### What is demand-controlled ventilation?

It varies the outdoor air supplied to a zone according to how many people are in it, often using a CO2 sensor as the occupancy signal. It suits spaces whose occupancy swings widely, such as meeting and training rooms. ASHRAE 62.1 sets conditions on its use, so whether and how it is applied in a project is for the design engineer to decide.

## Sources

- [ASHRAE 62.1](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2) — Ventilation and acceptable indoor air quality - ventilation rate procedure
- [NBC 2016, Part 8](https://www.bis.gov.in/standards/national-building-code/) — Building services - air conditioning, heating and mechanical ventilation
- [ECBC 2017](https://beeindia.gov.in/) — Comfort systems and controls

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Source: https://hagerstone.com/blog/office-co2-and-fresh-air-ashrae-62-1