ASHRAE 62.1 vs WHO
CO₂ guidelines,
explained for Indian buildings

Almost every article about indoor CO₂, including ours, quotes 1,000 ppm and attributes it to ASHRAE and the WHO. That is a reasonable shorthand, but the underlying documents say something more precise and rather more useful — particularly if you are applying them to an Indian building, where one of the assumptions behind the number is different.

What ASHRAE 62.1 actually specifies

ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality, is the international reference for ventilation in commercial and institutional buildings. It is worth stating clearly what it contains: outdoor-air flow rates, not carbon dioxide concentrations.

The core of the standard is the Ventilation Rate Procedure, which calculates the required outdoor air for a space as the sum of two terms:

Vbz = Rp × Pz + Ra × Az

Outdoor air required = (rate per person × number of people) + (rate per unit area × floor area)

The per-person term covers what the occupants themselves generate — CO₂, moisture, odour. The per-area term covers what the building generates — off-gassing from furnishings, carpets, finishes. For an office, the values are typically 2.5 L/s per person plus 0.3 L/s per m². For a classroom, 3.8 L/s per person plus 0.6 L/s per m², reflecting higher occupant density and different activities.

Nowhere in that is a ppm figure. So where does 1,000 come from?

Where the 1,000 ppm figure comes from

It comes from working the arithmetic backwards, and it originates in an appendix rather than the body of the standard.

If you supply outdoor air at roughly 7.5 L/s per person, and each person generates CO₂ at the rate a seated adult does, the room reaches a steady state at about 650 ppm above the outdoor concentration. That specific ventilation rate was chosen historically because it is approximately the rate at which the large majority of visitors entering a space judge the air acceptable — a body-odour perception threshold, established in the classic olfactory studies of the 1980s.

So the real rule is: indoor CO₂ should sit no more than about 650 ppm above outdoor CO₂.

The 1,000 ppm figure is that rule with an outdoor value of 350 ppm substituted in — which was roughly correct when the guidance was written. It no longer is.

Which matters in India

The global background is now around 420 ppm and rising, and urban Indian ambient air commonly sits a little above that — 430 to 450 ppm in a city, higher near heavy traffic.

Applying the differential correctly: 430 + 650 = about 1,080 ppm. So the practical target is a little above the quoted 1,000 rather than a little below it. This is not a large correction, but it has two useful consequences:

• If your building sits in a location with elevated ambient CO₂ — beside a busy road, in a dense commercial area — measure your own outdoor baseline before setting thresholds. Chasing 1,000 ppm indoors when outdoor air is 480 ppm means demanding far more ventilation than the standard intends.
• The differential is the honest metric. Indoor minus outdoor is what your ventilation is responsible for. It is the number an engineer should be looking at.

What the WHO says, and how it differs

WHO guidance on indoor air comes at the question from a public-health rather than a comfort-engineering direction, and its main indoor-air work concerns pollutants with direct health effects — particulate matter, nitrogen dioxide, carbon monoxide, formaldehyde, radon. Carbon dioxide at indoor levels does not belong in that category, and the WHO does not treat it as a toxicant at concentrations found in buildings.

Where CO₂ enters WHO and allied public-health guidance is as a ventilation indicator — most prominently in guidance on reducing airborne transmission risk in shared indoor spaces, where CO₂ monitoring became a recommended practical check on whether a room is being adequately ventilated.

Different reasoning, similar destination: keep occupied spaces below roughly 1,000 ppm, and treat sustained readings well above that as evidence of inadequate outdoor air.

The Indian codes

The National Building Code of India (NBC 2016), Part 8 specifies ventilation requirements for building types using the same per-person outdoor-air-rate logic, and Indian practice via ISHRAE generally aligns with ASHRAE 62.1 for commercial buildings. The engineering framework is common; what differs is enforcement and, more importantly, whether anything verifies that the design rate is being delivered in practice.

That last point is where CO₂ measurement earns its place. A building can be designed to 62.1, commissioned to 62.1, and still be running at 2,200 ppm three years later because a damper is shut, a fan has failed, occupancy has doubled, or somebody turned the fresh-air unit off to save power. The design rate is an intention. CO₂ is the measurement.

The one number that is not a comfort target

Worth separating clearly, because it appears in the same conversations: 5,000 ppm is the occupational exposure limit for carbon dioxide averaged over an 8-hour working shift (ACGIH TLV-TWA, matching the OSHA PEL), with a 15-minute short-term limit of 30,000 ppm.

These are safety limits for industrial environments — cold stores, fermentation spaces, CO₂ plant rooms, enriched grow rooms. They are five times higher than any comfort target and they are not a benchmark for an office. Anyone arguing that a meeting room at 2,000 ppm is fine because the limit is 5,000 has confused a toxicological threshold with a performance one.

What to actually set

Bringing it together, for a building in India:

None of these are in any standard. They are practical settings derived from the standards' logic, and every one of them is adjustable on the unit's front panel because the right value depends on your room, your ventilation capacity and your local outdoor baseline.