An OHS professional can often gather enough simple information to quickly provide approximate answers to questions about airflow in a space, regardless of the complexity of the system.

Every occupational health and safety professional must be able to evaluate the air the occupants of a space are experiencing to assess the potential for IAQ problems and their solutions.
Most OHS professionals today are unable to conduct

*in-depth*testing or measurement of HVAC systems and their airflows. Specialized knowledge of testing, measurement, and balancing is often required on the complex systems of today. Industrial hygiene engineers or TAB (testing, adjusting, and balancing) specialists can be employed to make detailed measurements. However, an OHS professional can often gather enough simple information to quickly provide approximate answers to questions about airflow in a space, regardless of the complexity of the system. This article provides guidelines for

*simple*testing, measurements, and approximations an OHS professional might perform. These include temperature and humidity; air movement and distribution, outdoor air flowrates, and air exchange rates in the occupied space; concentrations of carbon dioxide in the air; and the effects of wind on the airflow through a building. The following equipment is needed to perform the simple tests and measurements described in this article: tape measure, thermometer, psychrometer, smoke tubes, and carbon dioxide monitor.

1. SMOKE AND VISIBLE TRACERS

Smoke tubes can be useful because smoke makes air visible. Nothing convinces people of air misbehaving more quickly than seeing smoke leaking through cracks around a closed doorway, air stratifying in layers, or air moving in the wrong direction. (Be sure no one is exposed to the smoke, however. It can be irritating to building occupants.) Smoke can also provide a rough estimate of air velocity using the following equation:To estimate the air velocity through a doorway, an OHS professional releases a quick burst of smoke at a doorway. Let’s assume the small smoke plume travels through the door a distance of about two feet in one second. Using the above equation, the estimated velocity in feet per minute would be:

Velocities in doorways, hallways, windows, and hoods are often estimated using this technique.

2. USING TEMPERATURES AND CARBON DIOXIDE

The following simple techniques using temperature or carbon dioxide measurements can provide approximate estimates of outdoor air delivery rates to a space (see Figure 1). In the following examples, OA = outdoor (fresh) air, SA = supply air from the air handling unit (AHU) to the space, RA = returning air from the space to the AHU, and MA = mixed air at the AHU as the RA and the OA mix together.
The approximate percentage of outdoor air in the supply air can be estimated by measuring the temperatures of the air and using the following equation:

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You can go to the AHU to get these measurements. (Take the building supervisor with you—safety first.) Note that when the outside air temperature is near the indoor temperature, this approach will not be accurate.
Once you have the percentage of OA, you can estimate the delivery rate by multiplying the percent OA by the design or actual SA flowrate.
Assume you have recorded the following summer temperatures using a simple thermometer: Tra = 76°F, Toa = 90°F, and Tma = 80°F. The percentage of OA would be:

It is also possible to approximate the percentage of outdoor air in the supply air by measuring carbon dioxide concentrations according to the following formula:

Note that this approach gives percent OA for the time and the location that the measurements are taken. These measurements can be taken in the space (at the supply and return registers) and outdoors. Be sure the AHU is on and operating correctly, and watch that your breath does not contaminate the CO2 samples. (This applies to all measurements and equations using carbon dioxide.)
Assume the following CO2 concentrations: Cra = 850 ppm; Csa = 790 ppm; and Coa = 410 ppm. The approximate percentage of OA in the supply air would be:

3. USING NATURAL CARBON DIOXIDE AS A TRACER GAS

Carbon dioxide can also be used as the “tracer gas” to estimate the amount of OA reaching a space in a building. During the day, CO2 builds up naturally in an occupied space from occupants’ exhalations. After everyone leaves, a continuing supply of OA will dilute the carbon dioxide. Knowing the initial and final concentrations and the time elapsed allows the use of an exponential purge formula to predict the amount of OA delivered to the space. Figure 2 and the following formula describe the situation: where ln stands for natural log, N is the number of air changes per hour, Ci is the initial CO2 concentration at the start of the test, Ca is the concentration after a period of time in hours (typically one-half hour), and Coa is the outside air concentration. (An explanation of N and how to convert it to airflow in cubic feet per minute appear below.)
Be sure the AHU system operates normally during the time of the test and that the occupants (the sources of carbon dioxide) have departed.
Let’s say the carbon dioxide concentration inside a building measures 1,200 ppm at 5:30 p.m., when all the people have departed. By 6:00 p.m., the concentration has been reduced to 600 ppm. The outside concentration is 400 ppm. We can use this information to determine the approximate number of air changes per hour:

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4. AIR CHANGES PER HOUR

Some ventilation standards are based on air changes per hour, usually designated by the letter N. The equation relating airflow rates and air changes per hour is:where Vol is the volume of the space in cubic feet, “60” converts hours to minutes, and Qoa is the air volume flowrate in cubic feet per minute.
Using this equation, we can determine the approximate airflow rate for the conditions described in section 3. Let’s assume the room volume is about 40,000 cubic feet:

5. ESTIMATING AIRFLOW FROM PEOPLE PRESENT IN THE SPACE

According to studies by ASHRAE, people at minimal levels of physical work (such as those in offices and classrooms) exhale (or “emit”) carbon dioxide at an average rate of about 0.012 cfm. The following equation makes use of this “emission rate” to estimate the approximate delivery rate, Qoa, of fresh air to the space where occupants are located:The term n is the number of people in the space served by the AHU. C is the concentration of carbon dioxide measured in or just at the entrance to the return air system after the concentration of carbon dioxide has reached a “steady state” condition, usually just before noon or quitting time, typically 700–1,200 ppm (see the upper part of the curve in Figure 2). Coa is the concentration of CO2 in the OA, usually about 400 ppm.
Using this equation, we can determine the volume flow rate of outside air being delivered to the space and the amount of air per person for an office of 45 people where C is 1,000 ppm just before noon and Coa is 400 ppm:

6. NATURAL VENTILATION

Ventilation can be provided by simply opening windows and letting the wind blow through the building. As shown in Figure 3, wind can create generous amounts of air exchange in a building. But how much is going through? And what is required to make this work? At a minimum, air can infiltrate by diffusion at any opening (such as open doors and windows). But for best results, openings must be located both on the upwind and downwind sides, there must be a clear path between the front and the back openings, and a wind must be blowing.
To approximate the air flowrate through the building from wind, use the following equation:where Q is air flowrate in cubic feet per minute, A is the open area in square feet (for example, windows and doors, either upwind or downwind, whichever is larger), Vw is the average wind velocity in mph, and Kw is an angle factor. Kw = 0.3 when the wind is not blowing perpendicular to the building; Kw = 0.5 when it is straight on the building. Note that wind speeds vary widely at any one location, so wind cannot be relied on for consistent ventilation of spaces where air contaminants must be controlled constantly.
For the building shown in Figure 3, assume an oblique wind of 8 mph and an open back door area of 30 square feet. The estimated airflow through the building due to wind is:

Assuming a building volume of 50,000 cubic feet, we can use the equation in section 4 to estimate N, the air change rate:

These simple tools can be valuable to any OHS professional concerned with IAQ issues—but keep in mind that the answers they provide are approximate, not definite. See my ventilation workbooks published by AIHA for sources, references, and more detailed information.
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D. JEFF BURTON, MS, PE, CIH

**(VS 2012), CSP (VS 2002),**is an industrial hygiene engineer with broad experience in ventilation used for emission and exposure control. He can be reached via

email

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*The Synergist*

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Simple Techniques for Assessing Airflow in Occupied Spaces

Six Ways To Approximate Airflow