Fonte (Source): Consulting-Specifying Engineer
Por (By): William Carson Judge, TBE, CxA, Bay to Bay Balancing Inc.
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Poor indoor air quality (IAQ) contributes to a substantial number of health problems. ASHRAE Standard 55-1992 addresses the most common factors related to the comfort of occupants within the space. This article outlines the guidelines.
The U.S. Environmental Protection Agency (EPA) defines indoor air quality (IAQ) as:
“The temperature, humidity, ventilation and chemical or biological contaminants of the air inside a building.”
This represents any condition inside the building that effects the health and comfort of the building occupants; including temperature, humidity, and the concentration of pollutants. ASHRAE defines Acceptable Indoor Air Quality as:
“Air in which there are not known contaminants at harmful concentrations by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.”
Effects of poor indoor air quality
Poor indoor air quality contributes to a substantial number of health problems. Studies have linked it to poor performance in both the school and work environments. Effects can be immediate and the result of a single exposure or delayed, not showing up for years. Short term and immediate problems may manifest as allergic reactions, headaches, fatigue or asthma. At the other extreme, years of exposure to Radon may result in lung cancer. A single exposure to asbestos may cause mesothelioma, a fatal lung disease.
Different segments of the population may react differently to exposure. The very young, the elderly and those with suppressed immune systems may be much more likely to succumb to disease as a result of exposure. This increases the significance of these issues in schools, nursing homes and hospitals.
Causes of indoor air problems
Pollution sources inside the building that release or off gas pollutants can include, but are not limited to: air fresheners, smoking, perfume, cleaning products, cooking or process by-products, propane forklifts, boiler combustion by-products and off-gassing of chemicals by furniture, carpets and building materials.
Examples of outside sources of pollutants being taken in to the building through outside air intakes or infiltration into the building can include: radon, pesticides, atmospheric pollution, carbon monoxide and other combustion by-products from vehicle traffic.
ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy, addresses the most common factors related to the comfort of occupants within the space and addresses temperature, radiation, humidity, air movement, temperature stratification and drift as well as factoring in the clothing and activity level of the occupants.The scope of indoor air quality extends to temperature, humidity and lighting within a facility.
The conditions outlined in the table are guidelines from the standard that satisfy the thermal comforts required by the standard.
In addition to comfort issues, extremes in temperature and humidity can also exacerbate other potential sources for IAQ problems. Higher temperatures increase the reactivity of chemicals and accelerate off-gassing of compounds from building materials and interior furnishings. Increased humidity raises the risk of microbial growth and proliferation.
Beyond the physical values of temperature and humidity, many other factors can contribute to the perception of comfort. Air movement also plays a large role in thermal comfort. The lack of air movement can create a sensation of hot/stuffy air. Increased air velocity on the skin accelerates evaporation of perspiration which increases cooling. The same higher level of air movement can induce a chill in others. The goal is to find the balance of these variables that will provide the client with the highest level of satisfaction.
Increased humidity is a major cause of mold growth within the building environment. In high humidity parts of the country, it can be a considerable detriment to indoor air quality. The conditions for mold to grow require three components: mold spores, a media to feed on and moisture. Mold spores are essentially everywhere and mold is capable of using most media as food sources, including drywall, ceiling tiles, carpet and wallpaper. That leaves the control of moisture as the only practical method to control the growth and proliferation of mold.
Control of indoor air quality issues
Control of indoor air quality is typically addressed at three levels. The first step is administrative controls. Some examples of administrative controls are:
- Making decisions which would prevent the source of the pollutant in the first place
- Having a no smoking policy or using a different process for an in-house procedure
- Choosing low emitting products for maintenance and cleaning
- Isolating some processes in a remote location
Whenever practical, administrative controls are the best solution.
There are times when because of financial cost or sheer practicality, an administrative control cannot be used; in such cases engineering controls provide the next best solution. Engineering controls utilize a line of defense to separate the sources of pollution from the occupants in the conditioned spaces. These controls may:
- Utilize pressure barriers such as those seen in fume hoods or in isolation rooms operating under a negative pressure.
- Employ physical barriers such as a glove box in a laboratory or special filtration procedures.
There are times when the contaminant in the space does not come from one point source. This can include the off gassing of chemicals from building materials, cleaners used to clean and wax floors and body odors from building occupants. To address issues such as these, the best solution implements the third level of control, dilution ventilation.
One such solution is to introduce large amounts of fresh air to dilute the concentration of the pollutant to a level where it does not pose a problem.
TAB related issues
It is not the test-and-balance (TAB) technician’s job to design the project, but there are many aspects of the job that can ensure the design intent is fulfilled and the building occupants are provided a healthy facility for their use.
Ventilation is probably the single item over which we have the greatest influence. Most buildings are designed with criteria based on ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, which has become the generally accepted standard for commercial buildings in the United States. This standard combines many parameters to designate the appropriate amount of fresh air for a given space, such as the number of occupants, square footage of the space, the intended use of the space, building schedules, etc. This replaces older references to a fixed cfm per occupant.
Many systems use carbon dioxide (CO2) levels as criteria to control outside air levels. CO2 occurs naturally in the atmosphere typically at levels around 400 parts per million (ppm). Many control systems are designed to increase the outside air when the CO2 level reaches a set value, typically 1000 ppm. CO2 itself is not a problem at these levels: OSHA sets their permissible exposure limit at 5000 ppm. CO2 is used instead as an indicator of the ventilation effectiveness. If CO2 levels are rising to the 1000 ppm level due to generation from the human occupants then it follows that other pollutant levels will rise to potentially unsafe levels.
Outside air flows should be set up so that they satisfy the requirement outlined in the project documents under all conditions. If outside air flow is set on a VAV system at 100% flow then the same system may not provide the design requirement in a minimum flow condition when all terminal unit boxes are satisfied or in a heating mode. While some control systems accommodate such conditions many do not. A failure to meet the design in all modes should be reported and corrective action should be implemented by the project team.
Rooms hosting large groups such as classrooms, conference rooms or training spaces often have intermittent occupancies. ASHRAE allows the outdoor air requirement to be based on average occupancy as long as the peak occupancy is for a period of three hours or less. There should never be less than 50% of the maximum.
Many systems are wired such that the fan shuts off when the system is satisfied. Under such conditions the system is not meeting the requirements of the project documents and corrective action is required.
Sometimes the outside air itself can cause problems. In warm, humid environments a constant volume unit when satisfied may shut down the coil at the thermostat while the fan continues to bring humid unconditioned air into the space. When the air hits cool surfaces in the space the moisture in the air condenses and provides an avenue for mold propagation. Outside air can also be the source for pollutants. Outside air intakes situated over loading docks may capture truck exhaust and bring it into the space. Exhaust fans discharging polluted air, if located near outside air intakes, may result in kitchen or sewer gas odors being carried into the occupied space. In the worst case you may be exhausting air from an isolation room or chemical fume hood which can then be carried back into the space, resulting in dangerous conditions for the occupants.
Building pressure for most facilities is designed to be neutral to slightly positive. There are exceptions to this such as laboratories, restrooms, etc. which maintain negative pressures by design. In most conditions the amount of outside air will exceed the amount of exhaust air for a given space; this ensures that slight positive pressure and minimizes the chance for infiltration.
An excess of air can lead to over-pressurization of the building and result in problems with doors closing and bubbles in roofing membranes.
Building changes can result in creating IAQ problems where there weren’t any previously. Often walls are moved around, creating areas with no return or poor air mixing. Problems such as these are frequently seen in tenant modifications in office buildings.