Indoor Environmental Quality

Indoor environmental quality describes the total quality of the internal environment as a human being experiences it: the air they breathe, the temperature and humidity they feel, the light they see by, the sound around them, and the space and layout they move through. Air quality, thermal comfort, light and views, and acoustics are the four big pillars; this lesson takes air, and the next takes the comfort trio.

It belongs squarely in a sustainability course for a simple reason set out in Lesson 0.1: a building that harms the people inside it is not sustainable in any real sense. Sustainability is often framed as harm to the planet, but the occupant is part of the system too. There is also a hard-nosed argument. In a typical office, staff salaries dwarf energy and rent combined - people are by far the most expensive thing in the building. A ventilation upgrade that costs a few rupees or cents per square metre per year and lifts cognitive performance by several percent pays for itself many times over. IEQ is where the human case and the business case for good design meet.

The danger is that IEQ and energy can appear to pull against each other. A naive path to a low energy bill is to seal a building tight and stop bringing in fresh air, because conditioning outside air costs energy. That trade is a trap: it saves energy by degrading the very thing the building exists to provide. The whole-systems answer, developed through this module, is to ventilate well and efficiently - heat-recovery ventilation, demand control, and good filtration - so you get clean air and low energy, not one at the expense of the other.

Almost every indoor air problem traces back to one thing: not enough fresh air, or the wrong air, in the wrong place. Ventilation is the deliberate supply of outdoor air to dilute and remove the pollutants people and buildings generate - carbon dioxide from breath, moisture, odours, and chemicals off-gassing from furnishings (Lesson 6.4).

Ventilation is measured two ways. Air changes per hour (ACH) counts how many times the room's whole volume of air is replaced each hour - roughly 4-6 ACH suits many occupied rooms, more for kitchens, labs and clinics. Per-person outdoor-air rate is the more design-useful figure: ventilation standards such as ASHRAE 62.1 call for something like 8-10 litres per second per person in offices and classrooms, and health-forward standards push higher. Under-ventilate and CO2, moisture and pollutants build up; over-ventilate and you waste energy conditioning air you did not need.

There are three broad strategies. Natural ventilation uses openable windows, cross-breezes and the stack effect - free, healthy and, in India's climates for much of the year, genuinely viable when the design courts it (see passive heating, cooling and ventilation). Mechanical ventilation uses fans and ducts for control and filtration where outdoor air is hot, cold, noisy or polluted. Mixed-mode switches intelligently between the two. The regenerative move is to design the building's form - its section, openings and depth - so that natural ventilation does as much of the work as the climate allows, with mechanical systems (ideally with heat recovery) filling the gap efficiently rather than running by default.

Carbon dioxide is the most useful single number in IEQ - not because it is very harmful at indoor levels, but because it is an excellent proxy for ventilation. People exhale CO2 constantly, so a rising concentration tells you fresh air is not keeping up. Outdoor air today sits around 420 ppm; a well-ventilated room stays below 800 ppm; above roughly 1000 ppm you should act, and by 1400 ppm and up occupants report stuffiness and drowsiness and studies show measurable declines in decision-making and cognitive test scores. A cheap CO2 monitor is the single best diagnostic a designer or occupant can own.

CO2 is only the marker, though; the real hazards are others. Particulate matter (PM2.5) - fine particles from traffic, cooking and, in much of India, ambient smog - penetrates deep into the lungs and is among the most damaging pollutants of all; good filtration (MERV 13 and above, or HEPA) is the defence where outdoor air is dirty. Volatile organic compounds (VOCs) off-gas from paints, adhesives, furniture and cleaning products (the whole subject of Lesson 6.4). Radon, an odourless radioactive soil gas, matters in some geologies. Biological contaminants - mould, bacteria, dust mites - thrive where damp and poor ventilation meet, which is why humidity control is an air-quality issue too. Designing for clean air means all three moves at once: dilute with ventilation, remove with filtration, and cut the source with healthy materials.

Air quality is not only about gases and particles; it is also about water in the air, and this is where sustainability, comfort and health knot together most tightly - especially in India's monsoon and coastal climates. Relative humidity that sits too high or too low both cause trouble. The comfortable, healthy band is roughly 40-60% relative humidity. Below about 30%, air feels dry, mucous membranes and eyes irritate, and some viruses survive longer; above about 60-65%, the risks flip toward the biological.

Persistently damp air and damp surfaces are the breeding ground for mould, dust mites and bacteria - the biological contaminants of Lesson 6.1's pollutant list. Mould is not merely unsightly: its spores and the compounds it releases trigger allergies, asthma and respiratory illness, and a chronically damp building is a chronically unhealthy one. The causes are usually a combination of moisture getting in (leaks, rising damp, rain penetration), moisture generated inside (cooking, bathing, drying clothes, breathing), and too little ventilation to carry it away - plus cold surfaces where humid air condenses.

The design responses are squarely architectural and preventive. Keep water out of the fabric with good detailing, flashings and a continuous, correctly placed vapour and weather strategy. Extract moisture at source - mechanical extract in kitchens and bathrooms is one of the highest-value air-quality interventions there is. Avoid cold spots (thermal bridges) where condensation forms, through continuous insulation. And ventilate, closing the loop back to the master control: the same fresh-air flow that dilutes CO2 and VOCs also carries away excess moisture. In hot-humid climates, dehumidification (often coupled with cooling) may be needed; in all climates, a building that manages its moisture is a building that stays healthy for its whole life rather than slowly growing a hidden ecosystem behind its finishes.

For a generation, ventilation was the invisible, value-engineered-away service - the first thing trimmed to save capital and energy. The COVID-19 pandemic changed that overnight. Once it was widely understood that the virus spread through the air indoors, ventilation and filtration moved from a footnote to a front-page design priority. Suddenly clients, schools and employers were asking about air changes, CO2 levels and filter grades. The lasting legacy is not fear but expectation: occupants now assume, reasonably, that a good building will give them demonstrably clean air.

That expectation was already crystallising in the healthy-building movement and standards like the WELL Building Standard and the air provisions of green-rating systems, which treat occupant health as a first-class outcome rather than a by-product of energy compliance. The evidence base underneath it is strong: landmark studies of workers in 'green' versus conventional and enhanced-ventilation offices found significant gains in cognitive scores when fresh-air rates rose and pollutants fell.

The design implications are practical and enduring. Provide generous, controllable outdoor air with heat recovery so it need not cost the earth in energy. Filter to suit the local outdoor air - a non-negotiable in polluted Indian cities. Fit CO2 sensors and, ideally, show occupants the reading, because visible air quality drives both trust and behaviour. And design for damp control and easy maintenance, because the healthiest system on the drawing board fails if filters are never changed. Clean air is not a luxury layered on top of a sustainable building; it is part of what makes a building worth sustaining.