Concern about indoor air quality (IAQ) and the study of air quality issues is a fairly recent phenomenon. Some of the earliest documented studies occurred in Scandinavia in the mid-1960s and were focused primarily on thermal comfort issues. For the first decade or so, IAQ studies primarily involved comparing indoor air to outdoor air. The level of outdoor pollution was a chief concern and the goal was to ensure that indoor air was of better quality than the outdoor air subjected to pollutants.
A typical IAQ investigation requires several steps:
• Planning –
* Gather background information about the building and its systems.
* Interview affected people – understand the complaints and symptoms and check for patterns as to where and when they occur.
* Set objectives.
* Determine the strategy to be employed.
• Gathering data – make necessary measurements throughout the building, possibly including temperature, humidity, CO2, CO, particles, VOCs, chemicals and bioaerosols.
• Analyzing the data – check for acceptable measurements to eliminate certain areas or suspected problems, as well as anomalies that directs to areas requiring additional focus (remember, there can be multiple problems).
• Reporting findings – all results indicating a need for corrective action should be reported.
• Offering assistance – prepare an IAQ management plan that includes setting policies and conducting routine measurements to ensure good air quality is maintained.
To help steer an investigation, the affected occupants should be asked questions related to symptoms, origin of symptoms, the time of occurrence of symptoms, the nature and place of occurrence and change in one’s activities, etc. Criteria for determining indoor air quality can be separated into two basic categories – comfort and health. The difference, of course, is the way in which humans are affected, and some criteria may influence both comfort and health.
Common measurable characteristics of comfort include temperature, humidity, air velocity (draft), ventilation, vibration and noise. Factors much harder to quantify yet able to impact perceived individual comfort include light glare, odours, physical space layout, proximity to other areas, clothing, activity, and ergonomics. Even emotional or psychological stress in the workplace or at home can contribute to a person’s feeling of comfort. When making a measurement, allow sufficient time for the instrument to capture a “stable” reading. If you move from a hot area to a cold area and quickly take a temperature measurement, for example, the accuracy of the reading could be subject to question.
Temperature
Temperature is one of the basic IAQ measurements that has a direct impact on perceived comfort and, in turn, concentration and productivity. According to ASHRAE Standard 55, the recommended temperature ranges perceived as “comfortable” are 73 to 79°F (22.8 to 26.1°C) in the summer and 68 to 74.5°F (20.0 to 23.6°C) in the winter. Measurements should be taken periodically at many areas of the building to be sure that air is distributed evenly and temperatures are consistent. TSI offers a number of instruments that measure temperature. These include IAQ monitors, thermohygrometers and multi-parameter ventilation meters.
Humidity
Too little humidity in a space may create static build-up and people will sense that their skin feels dry. Too much humidity and people will think it feels “sticky.” According to ASHRAE Standard 55, indoor humidity levels should be maintained between 30 to 65% for optimum comfort.
Humidity can be measured in several ways. Typically, references such as relative humidity, wet bulb, dry bulb, humidity ratio and absolute humidity are used. Whichever method is chosen, measurements should be taken periodically and spread throughout the building to ensure that air is distributed evenly and humidity levels are consistent and within goals. Several portable instruments that measure humidity include IAQ monitors, thermohygrometers and multi-parameter ventilation meters. ASHRAE Standard 55 links temperature and humidity together to provide a measure of thermal comfort. The objective should be to set the appropriate temperature and humidity levels so as to maximize occupant comfort while controlling energy consumption.
Velocity
A quick spot check at the supply diffuser will show if sufficient air is entering a space. This will assure there are no unexpected blockages in the air system, such as a closed damper. Velocity is also a good indicator that air is being appropriately distributed or balanced throughout the building and reaching all the intended spaces. Measurements should also be taken in the actual occupied “zones” to assess how air velocity affects individuals. The instruments used are air velocity meters, rotating vane anemometers and the multi-parameter ventilation meters.
Volume
ASHRAE Standard 62 lists recommended outdoor air requirements expressed in terms of Cubic Feet per Minute (CFM) per person depending on the type of space and activity. The percentage of Outdoor Air must be calculated (see ventilation section). This percentage can then be multiplied by the measured airflow to calculate the amount of outdoor air being supplied. Air volume or flow into an area affects the air change rates or exchange of air between outdoors and indoors. This results from leakage in natural or mechanical ventilation systems. The exchange of air can have a large impact on indoor air quality as it may increase the amount of outdoor pollutants being introduced or, conversely, dilute and help remove contaminants generated indoor.
The average air velocity can be determined using a straight average for both round and rectangular ducts using the log-Tchebycheff method, a method that accounts for velocity losses due to friction. As the figures indicates, velocity measurements should be taken at a minimum of 25 points for rectangular ducts and, for round ducts, symmetrically disposed diameters with at least 6 points on each should be used. For the greatest accuracy, take these measurements at least 7.5 diameters downstream or 3 diameters upstream from any disturbance such as an elbow, venturi or take-off. ASHRAE Standard 111 has additional details on measuring flow in ducts.
To determine volumetric flow rate, the average measured air velocity is multiplied by the cross-sectional area of a duct. For example, if a duct is 2ft by 2ft (cross-sectional area = 4sqft) and the average measured air velocity is 150ft per minute, the resulting flow rate is 150ft/min x 4sqft or 600 CFM. Multi-parameter ventilation meters are capable of calculating automatically the volumetric flow rate when the cross-secti
onal area of a duct is entered.
An air capture hood can also be used to determine air flow. Capture hoods provide quick, direct measurements of air flow from diffusers, vents or grills. They are capable of collecting and storing real-time flow measurements and they are also valuable when balancing the system for proper flow in all areas. Air velocity meters, multi-parameter ventilation meters and air capture hoods all provide a fast, accurate means for measuring volumetric airflow.
Ventilation
The introduction of outdoor air helps dilute unwanted pollutants and gets them out of the building faster. ASHRAE Standard 62 presents recommendations pertaining to ventilation, or the amount of outdoor air introduced into a given area. It recommends a minimum volume per person over time, depending on the type of space and activity being performed, expressed in cubic feet per minute per person.
A good indicator of proper ventilation is the level of CO2 present in a space. Carbon dioxide is a normal by-product of respiration, combustion and other processes. Elevated levels of CO2 may indicate that additional ventilation is required. ASHRAE Standard 62 recommends an indoor level not to exceed about 700 ppm above outdoor ambient air which is typically about 300 to 400 ppm. Under normal conditions, even elevated CO2 levels are rarely a health hazard since levels up to 10,000 parts per million can be tolerated without ill effects by healthy people. Measurements should be taken between different areas, in air distribution zones, at varying heights and between indoor and outdoor areas to ensure that the building is properly ventilated.
Health and Safety issues
While comfort is important in maintaining productivity and concentration, many unwanted airborne contaminants can actually pose a threat to human health. Unhealthy IAQ conditions occur whenever vapours, gases or airborne particulates are present in concentrations that adversely affect one or more occupants of a space. Potentially toxic, infectious, allergenic, irritating or otherwise harmful substances are almost always around us. Usually they exist in such small concentrations that stay below a “trigger” threshold and get little attention. When concentrations rise above the threshold, problems can arise. Even at relatively low concentrations, some individuals are very sensitive to certain substances and may react adversely even though other area occupants are not bothered. In very extreme cases, concentrations may be high enough to be fatal to all occupants. Dangerous airborne substances are serious matters and must be dealt with, proactively, before problems get out of control.
Carbon Monoxide
The US EPA has set National Primary Ambient Air Quality Standards for Outdoor Air to be used in locating ventilation sources for buildings. Exposure limits for CO are an average of 35 ppm for one hour, not more than one time per year, or 9 ppm over any eight-hour period. The American Conference of Government Industrial Hygienists (ACGIH) and US Occupational Safety and Health Administration (OSHA) have also set maximum exposure limits in the Industrial Workplace Standard.
Measurements of carbon monoxide should be taken periodically and spread throughout many areas in a building to be sure that air is being distributed evenly and no dangerous levels of CO are detected. Pay particular attention to areas in which any form of combustion takes place. Typical examples of outdoor CO sources in a building include vehicular emissions from traffic or parking areas and building exhaust stacks. Indoor sources include furnaces, boilers, stoves and smoking areas. Instruments that measure carbon monoxide in real time include the IAQ monitors and combustion analyzers.
Airborne Particles
Respiration of particles challenges the body’s natural defence mechanisms and overexposure may strain these mechanisms, causing an adverse reaction. Inhalable particles are typically defined as those with an aerodynamic diameter of 10 micrometers or smaller, commonly referred to as PM10. Respirable particles, or those that readily enter the lungs, are usually classified as less than 4 microns in diameter. Sources may include dust, mists, fumes, smoke, environmental tobacco smoke (ETS) and other particulate by-products of combustion. ASHRAE Standard 62 recommends a maximum exposure limit for PM10 particles of 0.15 mg/m3 for a 24-hour average and 0.05 mg/m3 for an annual average exposure. This is consistent with the EPA’s National Ambient Air Quality Standards. The industry is moving in the direction of concern for smaller particles since they bypass natural defence mechanisms more readily and make their way deep into the lungs.
Three types of instruments – photometers, optical particle counters and condensation particle counters – normally are used for real-time measurements. Performance features and applications for the three are compared in the following charts. The specific instrument of choice depends on the application and the desired results.
Ultrafine Particles
Ultrafine particles (UFPs), defined as particles less than 0.1 micrometer diameter, are often produced by combustion and some chemical reactions. They are so small that they can pass easily through the body’s natural defence mechanisms to the deepest areas of the lungs. Certain people are extremely sensitive to ultrafine particles, sometimes regardless of chemical composition. It is suspected that the sheer number of particles and their cumulative surface area may trigger a reaction in these people. The only practical instrument for detecting ultrafine particles is a condensation particle counter (CPC), a device that “grows” the small particles to a size large enough to be counted using conventional particle counting techniques. The counter employs CPC technology to detect and track ultrafine particles within the building environment.
The method for tracking UFPs begins outdoors where several measurements are made with the particle counter to establish a baseline. If the building’s intake air is filtered, you can subtract from the base-line measurement percentage of particulates roughly equal to the efficiency rating of the filter to establish an indoor goal. For example, a 75% efficient filter effectively removes about three-quarters of all particles leaving 25% of the outdoor reading as the goal. Inside, measurements are taken and compared to this indoor goal. Seek levels of ultrafine particles greater than the goal to find sources of particles that might contribute to air quality problems. A basic understanding of the ventilation system and how outdoor air is introduced, filtered and distributed throughout the building is necessary for an effective investigation.
If levels of ultrafine particles significantly higher than expected are found anywhere in the building, take steps to locate and identify the source. Using the particle counter much like a Geiger counter, ultrafines can be traced quickly and easily directly to their source. Once a source is located, remedial action to control, repair or remove it is often straightforward. Another important parameter to consider along with ultrafine particles is differential air pressure. Airborne particles travel along seen and unseen pathways and are driven by air movement and pressure differential. Small particles naturally migrate from areas of higher to lower relative pressure.
Bioaer
osols
Some of these bioaerosols contain dangerous toxins that in extreme cases can cause a range of adverse health effects, including death. Besides serious diseases, some bioaerosols can also cause varying levels of irritation in certain individuals, including allergic reactions, headaches, eye irritation, sneezing, fatigue, nausea, difficult breathing and more.
Most biological growth requires some kind of food and water. Condensation, plumbing leaks, roof leaks, or even improper housekeeping can lead to unwanted moisture which can foster unwanted growth that must be checked and corrected. At this time, bioaerosols such as molds, fungi and bacteria must be collected, cultured and analyzed in an environmental microbiology laboratory setting to determine exactly what they are and how large of a presence they have. Sampling often consists of collecting material through an air sample on different sized filter media. In commercial and residential environments, “settle plates” and surface swabbing are not viable means of testing for biologicals. These methods were developed for testing in highly controlled environments and may grossly understate or overstate the condition in commercial and residential environments.
Some of the tools available to measure include electrochemical and infrared (NDIR) gas sensors designed to identify particular gases present in industrial settings, from combustions, emissions and other situations that could impact air quality. Photo-ionization and flame-ionization detectors can be used to identify many VOCs that can impact IAQ. In most cases, it is difficult to get an accurate picture of the extent of chemical contaminants in the air using real-time data collection. It is more often a complex mix rather than individual compounds that pose the difficult challenge. Consequently, sampling is an accepted practice generally conducted using techniques such as filtration, absorption in another media, or impaction.
It is important here to recognize that a healthy, productive working or living environment consists of more than just good quality air. The entire picture must be considered in order to optimize occupant satisfaction and productivity.
Indoor Air Quality Handbook, ASHREA