By Eamonn Ryan
Indoor air quality (IAQ) has never been static. From humanity’s first encounter with fire to today’s debates over ventilation standards, IAQ has reflected a persistent tension between comfort (what occupants can smell or tolerate) and health (what harms them over time).
The 2025 revision of ASHRAE Standard 62.2 incorporates this harm-based framework. Rawpizel/Freepik
As Max Sherman argues in IAQ Paradigms – The Next Generation, the field now stands at the threshold of a major paradigm shift: reframing ventilation not around odours or compliance, but around quantifiable harm.
Early lessons: survival and observation
The first IAQ challenge arose when humans brought fire indoors. Smoke accumulation in caves and shelters made ventilation a matter of survival. Long before formal standards, airflow meant the difference between health and illness.
Ancient civilisations intuitively applied ventilation principles. The Minoan palaces of Crete incorporated airflow paths resembling modern passive systems. In ancient Egypt, records describe overseers noting that indoor stoneworkers died younger than outdoor labourers – prompting practical ventilation solutions, even without scientific explanation.
During the Industrial Revolution, factory owners observed that better ventilation reduced illness and absenteeism while improving productivity. Ventilation became linked not only to tolerability but to workforce health and economic performance – helping inspire professional bodies such as ASHRAE and CIBSE.
The comfort paradigm
Early ventilation standards focused primarily on odour control. In the 19th century, physician David Boswell Reid and engineer Thomas Tredgold helped quantify ventilation needs. Tredgold’s work on the English Parliament established one of the first numerical ventilation rates – 4CFM per person – selected explicitly to control odours.
This comfort-based paradigm equated acceptable smell with acceptable air quality.
By the late 19th century, health concerns began reshaping thinking. Florence Nightingale’s hospital reforms showed that higher ventilation reduced infection and mortality. Ventilation rates rose dramatically, reaching 30CFM per person.
Yet by the mid-20th century, advances in medicine created a perception that infectious risks were under control. Ventilation standards retreated toward odour-based justifications. Ten CFM per person became common practice and was codified in ASHRAE Standard 62 (1973).
History reveals repeated cycles: crises elevate ventilation standards; complacency lowers them.
The harm paradigm emerges
By the late 20th century, IAQ faced conceptual confusion. Energy efficiency pressures reduced ventilation, while growing awareness of indoor pollutants argued for more. Andy Persily described the era as “ventilation for no particular reason”.
A pivotal shift came when ASHRAE reaffirmed that health must remain central to its mission. “Acceptable IAQ” was redefined to include two equal components:
- Comfort (odour and sensory acceptability)
- Health (avoidance of adverse effects)
Traditional standards relied on a hazards approach: set maximum concentrations for individual contaminants. But occupants face mixtures, not isolated exposures. Meeting individual limits does not guarantee minimal overall harm.
Since 2000, public health research has introduced a harm-based paradigm using Disability-Adjusted Life Years (DALYs), enabling comparison of different health outcomes on a common scale.
Research findings are striking: of roughly 45 commonly considered indoor contaminants, only a few drive most health impacts in homes:
- Fine particulate matter (PM₂.₅)
- Formaldehyde
- Nitrogen dioxide (NO₂)
Particles account for the largest share of harm – and are also the easiest to control through filtration, offering a rare opportunity to improve health while reducing energy use.
Standard 62.2 and the future
The 2025 revision of ASHRAE Standard 62.2 incorporates this harm-based framework. It establishes:
- A minimum ventilation rate (~7.5 CFM per person) for comfort.
- A harm budget, requiring additional ventilation or filtration only when health limits are exceeded.
Comfort sets the floor; health determines when more is necessary.
The harm paradigm also creates a pathway for integrating infection risk into routine design. With ASHRAE Standard 241 addressing epidemic conditions, infectious disease may soon be evaluated using the same DALY-based framework as chemical pollutants.
After centuries of oscillation between odour and illness, IAQ is entering a measurable, health-centred era – where ventilation is guided not by what we can smell, but by what truly harms us.
Source: Max Sherman, ‘IAQ Paradigms – The Next Generation’
