Carbon dioxide (CO2) is a byproduct of combustion, as well as a result of the metabolic process in living organisms. Because carbon dioxide is a result of human metabolism, concentrations within a building often are used to indicate whether adequate fresh air is being supplied to the space. Moderate to high levels of carbon dioxide can cause headaches and fatigue, and higher concentrations can produce nausea, dizziness, and vomiting. Loss of consciousness can occur at extremely high concentrations. To prevent or reduce high concentrations of carbon dioxide in a building or room, fresh air should be supplied to the area.

Carbon Dioxide
At room temperature, carbon dioxide is a colorless, odorless, faintly acidic-tasting, non-flammable gas. Carbon dioxide is a byproduct of normal cell function and is removed from the body via the lungs in the exhaled air. Carbon dioxide is also produced when fossil fuels are burned.

Surface soils can sometimes contain high concentrations of this gas from decaying vegetation or chemical changes in the bedrock. Depending on the temperature and pressure, carbon dioxide also can exist as a liquid or a solid. In its solid form, carbon dioxide is called dry ice.

Exposure to carbon dioxide can produce a variety of health effects. These may include headaches, dizziness, restlessness, a tingling or pins or needles feeling, difficulty breathing, sweating, tiredness, and increased heart rate 

Carbon dioxide levels and potential health problems are indicated below:

• 250-350 ppm: background (normal) outdoor air level
• 350-1,000 ppm: typical level found in occupied spaces with good air exchange
• 1,000-2,000 ppm: level associated with complaints of drowsiness and poor air
• 2,000-5,000 ppm: level associated with headaches, sleepiness, and stagnant, stale, stuffy air; poor concentration, loss of attention, increased heart rate and slight nausea may also be present.
• >5,000 ppm: This indicates unusual air conditions where high levels of other gases also could be present. Toxicity or oxygen deprivation could occur. This is the permissible exposure limit for daily workplace exposures.
• >40,000 ppm: This level is immediately harmful due to oxygen deprivation.

Sick Building Syndrome
Sick building syndrome (SBS) is used to describe a situation in which the occupants of a building experience acute health or comfort-related effects that seem to be linked directly to the time spent in the building, though no specific illness or cause can be identified. Building occupants complain of symptoms associated with acute discomfort (e.g., headache; eye, nose, or throat irritation; dry cough; dry or itchy skin; dizziness and nausea; difficulty in concentrating; fatigue; and sensitivity to odors).

When building designers started to make buildings more airtight with less outdoor air ventilation in order to improve energy efficiency, it was found the ventilation was inadequate to maintain the health and comfort of building occupants. The amount of carbon dioxide in a building is usually related to how much fresh air is being brought into that building; in general, the higher the concentration of carbon dioxide in the building, the lower the amount of fresh air exchange.

Consequently, analyzing levels of carbon dioxide in indoor air can reveal whether the heating, ventilation, and air conditioning (HVAC) systems are operating within standard guidelines. In approximately 500 indoor air quality (IAQ) investigations in the last decade, the National Institute for Occupational Safety and Health found that 52 percent of the indoor air quality problems were related to inadequate ventilation.

In order to have an acceptable indoor air quality with a minimum energy consumption, The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) revised ventilation standards to minimum outdoor air flow rates to avoid the problems related to inadequate ventilation.

ANSI/ASHRAE Standard 62.1-2013 Ventilation for Acceptable Indoor Air Quality
The purpose of this standard is to specify minimum ventilation rates and other measures intended to provide acceptable indoor air quality and that minimize adverse health effects. It is intended for regulatory application to new buildings, additions, and changes to existing buildings and to be used as a guide for the improvement of indoor air quality in existing buildings. This standard applies to all spaces intended for human occupancy, except those within single-family houses, multifamily structures of three stories or fewer above grade, vehicles, and aircraft. Ventilation requirements of this standard are based on chemical, physical, and biological contaminants that can affect air quality. Additional requirements for laboratory, industrial, health care, and other spaces may be dictated by workplace and other standards, as well as by the processes occurring within the space. First published in 1973, the standard is updated regularly and has undergone some key changes over the years, reflecting the growing body of knowledge, experience, and research related to ventilation and air quality.

Demand Control Ventilation
A demand control ventilation (DCV) system is an integral part of a building's ventilation design. It adjusts outside ventilation air based on the number of occupants and the ventilation demands that those occupants create. It is a ventilation system capability that provides for the automatic reduction of outdoor air intake below design rates when the actual occupancy of spaces served by the system is less than design occupancy. The ASHRAE Standard 62.1-2013 permits an HVAC system to change the outdoor air intake as operating conditions change based on occupancy. There are several ways to assess ventilation demand: occupancy schedules, which allow a building automation system to predict the current population based on the time of day; occupancy sensors, which detect the presence or number of people in each monitored zone; and carbon dioxide sensors, which monitor the concentration of carbon dioxide that is produced continuously by the occupants and diluted by the outdoor air. Regardless of which method is used, DCV strategies attempt to vary the outdoor-air intake in response to the current population.

When a space is vacant, there is no occupant-related pollution, so the occupant-related ventilation rate is not needed. Many types of high-occupancy spaces, such as classrooms, multipurpose rooms, theaters, conference rooms, or lobbies, have ventilation designed for a high peak occupancy that rarely occurs. Ventilation can be reduced during the many hours of operation when spaces are vacant or at lower than peak occupancy.

Components that control outside air are already required in most systems. These components can include an economizer or air makeup unit with modulating dampers. Other components needed for DCV are control sensors to measure occupancy and a controller programmed to communicate either directly with the economizer controller or with a central control system. Though occupancy can be measured in several ways, carbon dioxide sensing is the most common method.

Carbon Dioxide Detection
Carbon dioxide gas detectors can provide detection and automatic ventilation control for conference rooms, classrooms, meeting halls, or similar applications. The outdoor concentration of carbon dioxide can vary from 250-400 parts per million (ppm) or higher in areas with vehicle high traffic or industrial activity. The indoor carbon dioxide level depends upon the number of people present, how long an area has been occupied, the amount of outdoor fresh air entering the area, and other factors. Carbon dioxide concentrations indoors can vary from several hundred parts per million to over 1,000 ppm in areas with many people present for an extended period and where fresh air ventilation is limited. Outdoor "fresh" air ventilation is important because it can dilute carbon dioxide in the indoor environment.

The amount of fresh air that should be supplied to a room depends on the type of facility and room. Ventilation should keep carbon dioxide concentrations below 1,000 ppm and create indoor air quality conditions that are acceptable to most individuals. Carbon dioxide gas detectors can utilize an automated background calibration program to set the clean air level on a regular basis. The detector will maintain accuracy if it is exposed to the "clean air reference value" roughly once per week. The reference value is the lowest concentration to which the sensor is exposed. This applies when used in typical indoor ambient air.

Summary
When building ventilation is reduced, energy is saved because it is not necessary to heat or cool as much outside air. Reduced ventilation though can result in higher levels of carbon dioxide, which may cause building occupants to experience headaches, fatigue, or other symptoms. Heating or cooling for ventilation air can be enhanced by a demand control ventilation system (DCV), which can save energy while providing a comfortable environment.

Carbon dioxide concentrations within a building are often used to indicate whether adequate fresh air is being supplied to the building. These DCV systems use carbon dioxide sensors in each space or in the return air and adjust the ventilation based on carbon dioxide concentration; the higher the concentration, the more people occupy the space relative to the ventilation rate. With a carbon dioxide sensor DCV system, the fresh air ventilation rate varies based on the number of people in the space, saving energy while maintaining a safe and comfortable environment.