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CO2 in Air – Why is my Air Stale and Stuffy?

by / Wednesday, 05 June 2013 / Published in Healthy Building Inspections & Testing
CO2 in the air

CO2 in the Air – Why is My Air Stale and Stuffy?

Carbon Dioxide and Its Relation to Your Interior Air Quality. CO2 in the Air.

Why is my air “Stale?”

Many of my clients often complain of either specific symptoms, general malaise, or “just not feeling right” when in a building they spend a significant amount of time in; such as their home or office.   As an IAQ professional, it can be quite a challenge to determine the root cause or causes of occupant complaints.

Rarely is an exact cause identified.  It is often the case that a number of contributing factors are acting together, acting in an additive or multiplicative manner.  Small amounts of many contaminants, each on their own at levels that would not cause symptoms or concern, can add up to cause occupant symptoms and complaints.

Tracking down each individual contaminant source and either eliminating, encapsulating, segregating, or filtering them is often highly impractical, if not impossible.  Usually the best and most effective method of eliminating or reducing these contaminants to an acceptable level is by dilution.  This is most easily accomplished by the introduction of fresh, outside air.

We will discuss how measuring and monitoring interior Carbon Dioxide levels can help identify well-sealed or “Stale” conditions, and how to use CO2 in the air to help determine how much ventilation is required and if current ventilation is effective.

Today’s modern, ultra-efficient, tightly sealed buildings often compound the issue.  The remodeling of older homes and buildings can often seal them quite tightly.  Buildings that were designed for a few occupants, such as a warehouse, have been remodeled as an office and now contain many more occupants than the original HVAC system was designed to handle.  Typically, the issue of ventilation and fresh air intake is inadequately addressed or not addressed at all.  This can allow contaminants to continuously build up in a sealed space.

CO2 in Modern Homes Can Be High - "Build Tight, Ventilate Right!"

CO2 in Modern Homes Can Be High – “Build Tight, Ventilate Right!”

The EPA, in “Building Codes and IAQ” states:

How energy is conserved through building practices can have profound importance to the health of occupants because the majority of building energy goes to conditioning the air that occupants breathe.  Buildings in the past had high air change rates that ensured that the pollutants generated indoors were constantly diluted with outdoor air. However, except when outdoor climate conditions are complementary to what is desired indoors, such air changes require energy to condition the air for health and comfort indoors. This energy burden can be significant during outdoor temperature and humidity extremes.

In general, the primary ways of conserving energy in buildings involves:

(1) Improving the energy efficiency of equipment and appliances and reducing their unnecessary use, and

(2) Improving the thermal performance of the building envelope and reducing air change rates to minimize the energy used to condition the indoor air. This latter function– reducing air change rates—puts energy conservation in conflict with providing adequate indoor air quality if it provides insufficient outdoor air to dilute indoor generated contaminants, or insufficiently replace air exhausted by exhaust fans, clothes dryers and combustion equipment. This is of concern in all buildings, but especially in homes where adequate ventilation has traditionally relied on infiltration or occupant use of windows.”

Passive ventilation, i.e. opening windows, is a very effective and often “green” (it does not use fans or other electrical or mechanical means of moving air) way to ventilate a building and thus dilute contaminants.  This method is only energy efficient when the outside air is at a comfortable temperature and humidity.  Thus, in conditions where the outside air is excessively cold or warm, dry or humid, other methods of ventilation may be necessary to achieve desired energy efficiency.

Fortunately, there are engineering solutions such as HRV’s (Heat Recovery Ventilator) and ERV’s (Energy Recovery Ventilator) as well as other ventilation options that are more thermally efficient than just opening windows.

How can I tell if my air is “Stale”? Measuring CO2 in the Air.

CO2 in the air

CO2 Measurement Outdoor

Now, how to determine if your space is extremely well sealed, stale or stuffy?

I like to use Carbon Dioxide measurements and monitoring to help determine the state of interior air, ventilation effectiveness and “stuffiness.”

What is Carbon Dioxide (CO2)?

Carbon Dioxide a heavy odorless colorless gas formed during respiration and by the decomposition of organic substances, as well as a byproduct of hydrocarbon combustion.

From an IAQ standpoint, CO2 in the air is a surrogate for indoor pollutants emitted by humans and correlates with human metabolic activity. Carbon dioxide at levels that are unusually high indoors may cause occupants to grow drowsy, get headaches, or function at lower activity levels. Humans are the main indoor source of carbon dioxide. Indoor levels are an indicator of the adequacy of outdoor air ventilation relative to indoor occupant density and metabolic activity.  Basically, the more occupants in a given sealed indoor space, the more quickly CO2 levels will increase, and conversely, Oxygen levels will decrease.  So interior CO2 levels are useful way to measure how “Stale” a given interior space may be.

NIOSH considers indoor air concentrations of carbon dioxide that exceed 1,000 ppm (parts per million in air) are a marker suggesting inadequate ventilation.

ASHRAE (American Society of Heating, Refrigerating and Air-conditioning Engineers) recommends that carbon dioxide levels not exceed 700 ppm above outdoor ambient levels.   (Normal range for outdoor levels are typically in the 350-450 ppm range)

OSHA limits carbon dioxide concentration in the workplace to 5,000 ppm for prolonged periods, and 35,000 ppm for 15 minutes.

I tend to use the NIOSH recommendations in my inspections and surveys.  If I measure CO2 levels above 1,000 ppm, I will take a closer look at the ventilation situation.  A one-time sampling can be deceptive.  Perhaps the building was empty when sampled, and normally there are 40 workers in the space.  Perhaps the windows are open on a nice day, when normally they are closed all winter.   I prefer data logging CO2, with measurements every 15 minutes, for at least 48 hours, a week preferable, to make sure multiple conditions are sampled.  If the average level of CO2 is around or above 1,000 ppm, remedial action may be recommended.

Monitoring Interior CO2 in the Air 

co2 in the air

CO2 Measurement Interior

There are several decent and not overly expensive (under $500) CO2 monitors available.  Some have features that will log readings over time or turn on a ventilation fan or system if CO2 levels climb above a user set concentration.

It is important to note that bringing in fresh air from the exterior is only beneficial if the outside air is clean.  Air pollution can be quite severe in some areas.  Outdoor air being brought into a building for occupant ventilation should at a minimum meet EPA NAAQS (National Ambient Air Quality Standards).  Many local Air Quality Management Districts, such as the Bay Area Air Quality Management District in my area around San Francisco, California, sample, measure and publish data about local exterior air quality and compare their results to the EPA’s NAAQS.

ASHRAE Standard 62.1 states that “Outdoor air not meeting EPA standards to be treated to prescribed levels for PM10, PM2.5, and ozone.”

If exterior air does meet EPA NAAQS, how much fresh air needs to be introduced into a “Stale”, interior environment?

How Much Fresh Air Do I Need?

The International Mechanical Code, IMC-2009, recommends 0.35 ACH (air changes per hour) and not less than 15 cfm (cubic feet per minute) of fresh air ventilation per occupant for a residential environment.  This can be accomplished through infiltration, passive or active ventilation:

CO2 in the air

Old World Style, Natural Ventilation

– Natural ventilation shall be through windows, doors, louvers or other approved openings to the outdoor air with ready access to and controllable by the building occupants.

– Openable area shall be at least 4% of the floor area, or mechanically ventilated with system capable of producing 0.35 ACH in the room, or whole-house mechanical ventilation capable of supplying 15 cfm per occupant.

– In HVAC systems, mixing outdoor air with return air is allowed.

– Return air must not be from identified contaminated spaces (e.g. bathroom).

– Supply and return air flow must be approximately equal.

ASHRAE Standard 62.1-2010 recommends in an office environment a minimum of 5 cfm per occupant. Outside Air must be delivered to occupants’ breathing zone at prescribed rates for all occupied spaces, during all hours of occupancy, and at full or part load conditions.


With today’s modern, well sealed buildings, it is becoming more and more important to be aware of how “Stale” our interior air is and to also be aware how important fresh air ventilation is to the health of occupants and to the importance of reducing via dilution the accumulation of interior air contaminants that that affect occupant health.  Monitoring CO2 levels is a fairly easy and inexpensive way to indirectly determine interior “Stuffiness” and ventilation effectiveness.




4 Responses to “CO2 in Air – Why is my Air Stale and Stuffy?”

  1. […] may cause occupants to grow drowsy, get headaches, or function at lower activity levels.  (  What’s the threshold of […]

  2. […] may cause occupants to grow drowsy, get headaches, or function at lower activity levels.  (  What’s the threshold of […]

  3. […] absorb carbon dioxide from the air and release oxygen in the process of photosynthesis. High levels of carbon dioxide or CO2 make a room feel “stuffy” and cause fatigue, poor concentration, and even headaches and […]

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