Fresh Air Dilution is the Solution

     Based on NIOSH studies and AET's field experience, greater than 50% of IAQ problems/complaints originate from inadequate fresh air ventilation into occupied spaces. As a result, the air becomes stagnant allowing odors and contaminants to accumulate. A general rule of thumb is that a pollutant concentration is reduced by about 50% for each doubling of the ventilation rate.

     AET's IAQ investigator's first step in evaluating the quantity of fresh air entrainment is to determine the outdoor air damper setting on the building's HVAC system. These dampers are normally located within the fresh air intakes on the roof of the building. Smaller HVAC units utilize manual or fixed dampers which are opened/closed manually and are sized to provide a specific air volume. Automated HVAC systems utilize mechanical actuators for damper operation; these systems open/close dampers according to preset temperatures and/or carbon dioxide (CO₂) levels.

Location, Filtration & Maintenance

     These three critical factors combined with the air pollution index in your area dictate the quality of fresh air introduced into your building. Outdoor air is drawn into the fresh air intakes, filtered, conditioned, mixed with return air and distributed throughout the building. Excess air helps maintain a positive pressure within the building and eventually passes out through building openings, doorways, or through exhaust systems.

Problems AET's IAQ Investigators look for:

1. Location of fresh air intakes should be on the roof or exterior wall well above ground level to minimize odors and contaminants from loading docks, garbage receptacles, vehicle traffic, etc.
2. Intakes should NOT be placed within 25 feet of sewer vents, roof exhausts, cooling towers (the further the better) or the sources delineated in #1 above. Intake location in conjunction with the prevailing wind direction may require extended distances to prevent re-entrainment of contaminants.
3. The bottom of the air intake should be at least 8 inches above the roof field or other horizontal surfaces to prevent blockage from leaves andsnow.Water drainage should be directed away from intakes to eliminate ponding water/debris near the intake.
4. Grills and intake screens must be periodically inspected and maintained. Waste from birds and other pests can disrupt proper operation of the HVAC system, generate odors, promote microbial growth and are potential pathogens. Vertical louvers, rather than horizontal louvers should be utilized to reduce the potential for roosting or nesting.
5. The filters in the HVAC unit must completely cover the intake area to prevent air flow bypass. Filters should be replaced according to an established schedule; the pressure differential across the filter bank measured with a gauge or manometer. Air filters should have a dust-spot rating between 30-80% or MERV rating of between 8 and 13. The higher the rating the better the protection from particulate for equipment and for occupants.

Return Air vs. Supply Air

     HVAC systems recycle building air to minimize energy costs. Normally, return air is hard ducted or directed via open plenum design and mixed with the unconditioned outside air from the fresh air intakes. A general rule of thumb is that there should be at least one return air vent in each room. The placement of return vents is how the building breaths and the pathways for how pollutants are transported through your building. Problems occur where limited return vents exist, when physical barriers obstruct air movement or where pollutants are present. Further, problems can occur when supply and return vents are too close together causing short circuiting of building air.

Building Pressurization/Exhaust Air

     Buildings should be designed to operate under a slight positive pressure relative to theoutdoors. The objective is to push out conditioned air, rather than draw in unwanted odors, vehicular exhaust, and other contaminants.

     Restrooms, smoking lounges, copy centers, print shops, chemical storage and other special use areas located within buildings should be designed to operate under negative pressure relative to adjacent spaces. These areas should also have a separate, dedicated exhaust system to the exterior of the building, which is adequately distanced from fresh air intakes.

     Your IAQ Investigations should include checking the building pressurization either visually utilizing smoke tubes or by measurements with micro-manometers.

What's That Smell? Carbon Dioxide

     CO₂ is a colorless gas which is naturally present in the earth's atmosphere at concentrations of about 330-390 ppmby volume. Elevated outdoor CO₂ concentrations can be found near vehicular traffic areas, industrial facilities and combustion sources. CO₂ is odorless except at high concentrations where it has a sharp acidic odor. The odor threshold for CO₂ is about 74,000 ppm; this concentration exceeds the IDLH level of 50,000 ppm. Thus, little or no warning isprovided in life threatening situations. The OSHA PEL standard is 5,000 ppm.

     The stale odors in commercial buildings, schools, etc. resulting from inadequate ventilation are not necessarily duetoCO₂ but are caused by a complex mixture of odors and other chemical pollutants. Based on AET's experience, conducting over 3,000 IAQ investigations in commercial buildings/schools,CO₂ levels normally do not exceed 2,000 ppm. AET's experience has only identified CO₂ levels exceeding the PEL during IH evaulations in CO₂ process areas (e.g.,bottling plants) and confined spaces.

CO₂ as an indicator for IAQ

     AET's IAQ investigators use CO₂ levels as a screening tool to evaluate the efficiency and effectiveness ofthe HVAC system. People inhale oxygen and exhale CO₂. The average breath contains about 35,000 to 50,000 ppm CO₂ (100x's greater than outdoor air). The CO₂ quicklymixes with the surrounding air and is diluted to harmless levels as long as adequate ventilation is present. AET's experience indicates indoor CO₂ levels exceeding 1,000 ppm or levelswhichare 700 ppm greater than the outdoor CO₂ concentration can result in increased IAQ-related complaints including headaches, fatigue and upper respiratory irritation. Properly ventilated buildings should have CO₂ levels at or below 800 ppm. 

ANSI/ASHRAE Standard 62.1-2007

     The rate at which outdoor air is supplied to a building is specified by building code whichis based on the facility construction date. Supply or ventilation rates specified in the building code are based on the need to control odors and human comfort. These ventilation rates are normally expressed as cubic feet of outdoor air per occupant (cfm/occupant).

     ASHRAE 62.1-2007 Ventilation for Acceptable IAQ is the most recent national consensus standard which is currently applied to building codes for the design and operation of ventilation systems. Minimum ventilation rates based on type of occupancy are listed in Table 6-1 of the Standard. For example, office spaces should have a ventilation rate of 17 cfm/occupant. School classrooms should have a ventilation rate of 13-15 cfm per child (varies on children's age).

     AET's IAQ investigators take critical measurements for CO₂ and calculate the actual cfm per person for direct comparison with the ASHRAE standard. In addition, outdoor air requirements for a specific room or area are calculated. For example, a typical classroom with 30 people would require aminimum of 15x30 or 450 cfm of outside air to be compliant with the ASHRAE standard.

Conclusion

     Supplying acceptable quantities of outdoor air to occupied spaces is a critical component of good IAQ. The IAQ investigator must be knowledgeable of the overall HVAC system design as well as the fresh air damper settings and control system. Measurement of CO₂ levels are a good screening tool to evaluate HVAC system performance. However, care must be taken as misleading results can be found in multi-use areas with variable occupancy rates such as conference rooms, auditoriums, etc. Meaningful data and decision-making should be based on multi-day CO₂ monitoring to ensure the building or occupancy zone is occupied long enough to allow CO₂ levels to reach abalance/equilibrium with the ventilation rate.

     Remember, in cases where an indoor pollutant is present, increasing fresh air or ventilation rate for the building only reduces exposure,while removing the source can completely eliminate exposure. Source control includes keeping pollutant sources out of the building through wise choice of furnishings and finish materials, using exhaust fans to capture and remove pollutants and controlling pressures between zones to keep pollutants from migrating to populated or sensitive areas.

     AET's building scientists and engineers are uniquely qualified to evaluate your building conditions and HVAC performance relative toindoorair quality. We offer immediate response and proven solutions to your IAQ needs including a critical balance with energy savings. Contact Roy Mosicant, CIH at 610-891-0114 or 800-9696-AET to get your free telephone consultation.

Alan Sutherland has been a Certified Industrial Hygienist since 1978 with over 30 years of CIH-related environmental consultingexperience. He has a Masters Degree in Environmental Science from Drexel University and is the founder/owner of Accredited Environmental Technologies, Inc. (In 1984).He is uniquely trained and licensed as an Environmental Professional in both the field and laboratory. He has been the founder of two AIHA Accredited Laboratories anda mentor to six (CIHs). Mr. Sutherland is also a Certified Hazardous Material Manager. He can be reached directly at 610-891-0114 or email a.sutherland@aetinc.biz.

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