Indoor Air Quality
ANSI/ASHRAE Standard 62.1

Indoor air quality is a complex issue. Pure earth atmosphere, at 70°F with 50% relative humidity and devoid of smoke particles, mold spores, air-bourne pathogens, plant pollen, respirable dust and with oxygen and carbon dioxide levels that remain constant despite our breathing - would be ideal, but terribly impractical. Compromises are needed, and the complex trade offs are the subject of the field of indoor air quality.

Attempts have been made to create universal guidelines which can be incorporated into building codes. Some techniques are simply prescriptive, using static formulas. If these calculations tell you that your house needs 100 ft3 of fresh air per hour, will this give you have a healthy place to live? The answer depends on whether the air outside is fit to breath and not contaminated with automobile exhaust, ozone, pollen etc. If so then you are trading one set of problems (low oxygen, high carbon dioxide, cooking fumes) for another (a code-purple air pollution day perhaps). For an asthmatic individual, one air exchange per hour could be great one day, and a mistake the next day. To mitigate this, standards require that the quality of the outdoor air be studied first before design decisions are made. While common in commercial buildings, this kind of analysis is seldom done for residential homes. A better approach is to determine and monitor contaminants of interest, and have a ventilation system that works to maintain the air within acceptable limits. The only limitation of this approach for home owners is that the measurement equipment is currently cost prohibitive, although this need not be the case because the technology to produce low cost sensors exists.

One of the more commonly cited references on indoor air quality (IAQ) is ANSI/ASHRAE Standard 62.1. ASHRAE is the American Society of Heating Refrigeration and Air conditioning Engineers. In the section below, we will analyze it in great detail and relate it to single family home constriction.

ANSI/ASHRAE Standard 62.1

This standard sets minimum ventilation rates for indoor or enclosed areas that have human occupants, and assumes that the outside air is free of "unusual" contaminants. Human comfort is not considered; for example, humidity levels that are uncomfortable aren't a problem unless they encourage the growth of mold.

They also acknowledge that what is acceptable for the majority (80%) of people, can be dangerous to sensitive populations (elderly, sick, young children); that odors are perceived differently by different populations and context sensitive (toilet odor in the kitchen may not be acceptable) and not considered by the standard; nor are outdoor pollutants like carbon monoxide.

This is simply a legal status. You can check out your area and drill down into the detailed maps to see how your building area is rated.

The goal here is simply to make a determination as to whether the outdoor air is to be classified as "attainment" or "non attainment".

The purpose of this is to ensure that the designers of the ventilation system are aware of air quality problems in the design phase, as it might determine the need for air filters, electrostatic precipitators, or special procedures.

Natural Ventilation

They mention that studies of naturally ventilated buildings (e.g. windows that open) have fewer "sick building syndrome" complaints overall. The guidelines are as follows:

For example: A 8'x8' kitchen with no windows opens to a living room 8'x14', and the living room has window on the far wall that opens 32" by 16". The total floor area of the two rooms is 176 sq ft. The window area window is 3.5 sq ft. The distance between the far ends of the living room and kitchen are 24 feet, which just makes the specification. However, if there is a kitchen door, then the kitchen is not permanently open and fails the specification. The window area is 1.9% of the total floor area, so it fails. The window is 3% of the living room floor area, so it fails too. To pass, the living room window needs to be a little larger and the kitchen needs a window or the kitchen door needs to go and the living room window needs to be about twice as large.

I will add a link later, but I suspect that many residential insect screens reduce the air flow significantly.

A large 36" doorway that is 8' high, is 21 square feet, so you need something more along the lines of an archway or completely open wall. In the kitchen example, 8% of the floor area is only 2.9 sq ft.

It is interesting that the 4% and the 8% in the equations above were chosen "to be consistent with model building codes in the United States". I would like to see gas diffusion studies to back up these figures as being optimum.

Engineered natural ventilation

The standards do not cover this area, but it is worthwhile noting that clever design can take advantage of thermal currents (heat rising up through a vaulted ceiling and out from a central vent), or other design features that provide a constant movement of air, resulting in a design that is not compliant with the ASHRAE standards, yet totally acceptable. Their take is that if the appropriate authority with jurisdiction signs approves, then it complies.

Exhaust Ducts

This is to make it impossible for exhaust air to leak back out into the living area. If the exhaust fan is at the building exit, on the distance between the fan blades and the exhaust louvers are pressurized. In the case of a residential heat exchanger, this is probably 4 feet of flexible ducting and many of these do indeed leak. If the exhaust fan is deeper inside the house, then the pressurized exhaust ductwork needs to be sealed in accordance with accordance with SMACNA Seal Class A.

The intake of a ventilation system must be keep away from source of pollution. This of course is obvious, the question is rather -- how far away?

Intake Separation Distances
Item meters
exhaust (smelly, irritating) 5
dangerous exhaust 10
chimneys and flues 5
automobile area, parking garage 7.5
parking lots, street, driveways 1.5
high volume traffic 7.5
distance from surface to intake, factoring in snow depth 0.30
Garbage dumpster 5
Cooling tower intake 5
Cooling tower exhaust 7.5

These types of things need to be adjusted based on a more through investigation. For example, if the smelly exhaust was from a curry house, stating that its not your problem because the intake is 5m away, is not likely going to be well received. This of course, should have been described thoroughly in the initial outdoor air quality report.

Rain Intrusion and Entrainment

This is to minimize the growth of mold, and needs serious consideration in colder climates. There are many well engineered rain hoods that are totally useless against blowing snow, and I have seen many residential air exchangers in Canada that are soaked much of the winter. The standard leaves the problem of snow entrainment up to the designer, which means for most residential owners of heat exchangers, they will need to fix the problem themselves.

Birds are a particular hazard because bird feces caries many diseases, and the are usually heavily parasitized with fleas and ticks which could be sucked into the intake.

Local Discharge

The ventilation of combustion appliances, such as heaters, is not addressed directly, other than specifying that local building codes and the manufacturer's instructions be adhered to.


This prevents dirt accumulation and the growth of unwanted things. Residential heat exchangers often fill with filth in this manner because of condensation and need to be washed out every few months. For commercial buildings, it would be an issue with cooling coils and evaporative coolers.


This is not a level designed for comfort, rather it is a point where condensation on cooler surfaces and subsequent mould growth is a possibility.

Obviously this is only possible if the difference is exhausted through cracks and leaks. The resulting mild pressurization prevents moist outside air from entering and condensing on cooler surfaces.

This is accomplished with pan slope and drain size. Residential heat exchangers have this designed into them, and have a drain tube that is inserted into a drain or to the outside. The tube usually is looped to create a water trap that creates a seal against drawing air in from the drain.

This makes sense because if is contaminated, after evaporation, the contaminates will be sent through the system as a power.

Building Envelope

Soil gasses like radon are not covered, and are likely covered by your local building codes.

A typical residential example in colder climates is the incoming domestic water pipe. The ground water is often quite cold, and water droplets form rapidly when the water is running, and wet the surfaces below which are often ceilings. They give an example of cold water pipes in a commercial bathroom as being exempt because they can be cleaned, and defer to local building codes.

Air Classification System

CLASS1 Nothing objectionable, can be recirculated (living room air)

CLASS2 Mildly offensive (pool or gym locker room)

CLASS3 Irritating and offensive (autopsy room and other horrible things)

CLASS4 Dangerous or harmful (kitchen hood, paint spray booth)

The last point is applicable to heat recover ventilators, such as units from Venmar that are common in northern homes. There are tight seals around the heat recovery core resulting in mixing far less than 10%. I would question the wisdom of mixing 5% autopsy air with the air in a living space.


The procedures for determination of adequate air-exchange rates are based on achieving an 80% satisfaction rate. This isn't a problem for a public space where the majority rules, but is a problem for a home owner that is one of the 20%, perhaps with allergies, and excellent sense of smell or low tolerance to sensory assault.

Ventilation Rate Procedure

One procedure is to treat the room type and occupants separately, and sum them together to get the flow rate needed by a room.

In the table below, the human component is categorized by the type of activity, and the rate in Liters per second required due to having people in the building.

Category L/s per person Description
0 0 A storage facility where human activity is negligible compared to the ventilation needs of the building.
1 2.5 Quiet office work
2 3.5 Lobby (more people around)
3 5 classrooms and schools
4 10 exercise rooms

These are the building components, categorized, with liters per second of air per square meter of floor space.

Category L/s m2  
0 .3 Conference room, lobby
1 .6 Classroom
2 .9 Art classroom with paints and glues
3 1.5 special cases
4 2.4 special cases

Breathing Zones

They assume that people are breathing the air in a zone that starts 3 inches from the floor, up to six feet (unless you are a basketball player), and two feet out from the walls. This is referred to as the breathing zone.

The significance is that ventilation air needs to flow through this zone, and not high up in a ceiling, to be effective. With cooling, this is not a problem because the cool air settles forcing the stale warmer air upwards. It can however be a problem in the heating season. In a residential setting, air mixing can be solved with ceiling fans.

Residential heat exchangers are not likely to be affected during the heating season because their outputs will be colder (having come from outside will always be cooler than the outgoing air), and drop to the floor.

VRP summary

The Indoor Air Quality Procedure (IAQP)

The previous procedure was formula based, whereas IAQP is based on controlling the concentrations of contaminants. This requires:

In a residential setting, the contaminants of concern are likely CO2, excessive humidity, cooking exhaust, and small particles. The concentration of total volatile organic compounds is an excellent indicator if you have a way to measure it.

There is a lot more in the specification, including scenarios specific to multi zoned HVAC systems in large commercial buildings that are unlikely to apply to the average home owner.


This specification has a lot of worthwhile considerations for the designer of residential dwellings, In particular. The following are things I would consider important:

Update: February 2011 - A new article on residential On-Demand ventilation has been added, where gas sensors are being used to continuously adjust the ventilation.