Indoor Air Quality in Office Buildings: A Technical Guide

5. Detailed Assessment

5.2 Individual Source Evaluation

5.2.4 Carbon Monoxide

Carbon monoxide is a colourless, odourless, toxic gas that is a product of incomplete combustion. Carbon monoxide pollution occurs where combustion gases are not properly exhausted or are being re-entrained into the building. Carbon monoxide should be measured if there are complaints of exhaust odours or if there is some other reason to suspect a problem with internal combustion gases.

In office and commercial buildings, important sources of combustion contaminants include tobacco smoke, garages, and loading docks that are attached or have a pathway to working spaces. Air intakes located at ground level or adjacent to vehicles or other combustion sources can transport contaminants to areas served by the air handling system.

ASHRAE Standard 62-1989 indicates that the 8-hour average exposure limit for carbon monoxide should not exceed 9 ppm. However, levels above 5 ppm indicate the undesirable presence of combustion pollutants - once located, they should be exhausted.

Carbon monoxide is extremely toxic. It combines with haemoglobin in the blood, reducing the oxygen supply to the body. At elevated levels, symptoms of exposure include headaches, decreased alertness, flu-like symptoms, nausea, fatigue, rapid breathing, chest pain, confusion, and impaired judgement. The degree to which these symptoms occur depends on health status and individual variations in sensitivity, so specific responses at a given concentration will vary among individuals.

5.2.4.1 Checklist

• The study of floor plans and/or discussions with the building operator may reveal possible problem areas.
• Inspect the loading dock and parking garages:
  
  
  • Are they properly ventilated? Is the exhaust blocked off?
  • Are trucks or cars left running?
  • Is there an access route from the dock or garage to other building areas?
  • Are stairways, elevator shafts, and ducts acting as pathways for automobile exhaust and diesel fumes?
  • Are carbon monoxide sensors (for ventilation control) and alarms installed in the garage calibrated and operating properly?
  • Is ventilation switched on only at peak periods?
  • Is a diesel generator located near an air entry route?
    
    
• Inspect the office area:
  
  
  • Are the occupants ill or complaining of symptoms such as headaches, fatigue, dizziness, and nausea?
  • Are the occupants working near possible sources of combustion products?
  • Are stoves and other sources fitted with exhaust systems?
  • Do the odours or symptoms occur during the peak hours of incoming and outgoing traffic?
  • Are air intakes close to a street carrying heavy traffic or to other exhausts containing combustion products? Outdoor air intakes located below third-floor level can conduct fumes from vehicular traffic, parking garages, and loading docks.

5.2.4.2 Measurement Methods and Equipment

Measurements should be made near sources, complaint areas, stairways, and elevators linked to sources.

a. Direct-reading tubes

Direct-reading tubes are a low-cost form of measurement used for spot sampling. A known volume of air is drawn through a detection tube by means of a hand pump. The length of stain is proportional to the carbon monoxide level and can be read directly (in ppm) from the detector tube. However, the measurements are not exact, and the detection limit should be less than 5 ppm.

Long-term sampling tubes with a sampling pump can be used to obtain an average concentration over longer periods.

b. Electrochemical analysers

Electrochemical analysers are small, compact detectors that provide immediate, accurate results and are useful for survey work or continuous measurements. They sample air by diffusion or by use of a small pump. The monitor employs an electrochemical cell, where carbon monoxide is oxidized to carbon dioxide, and generates a proportional electrical signal.

These devices are less expensive than infrared analysers, are easy to use, and can operate for long periods of time on standard batteries. They require calibration for zero and span concentration.

c. Infrared monitors

Infrared monitors are light, direct-reading portable units. They are usually more expensive than electrochemical monitors. Some electrochemical and infrared monitors can store data, which can then be compared with traffic load.

The target gas is sensed by its characteristic absorption line in the infrared region of the electromagnetic spectrum. The detector generates an electrical signal output based on the difference in absorption between a reference and a sample cell.

5.2.4.3 Strategy for Remediation

Carbon monoxide levels higher than 5 ppm can indicate a problem. Problems can usually be mitigated by:

• ensuring that offices adjacent to parking garages and loading docks are under positive pressure
• increasing ventilation to the problem area
• closing pathways between the contaminated area and the occupied space, ensuring that doors are well sealed
• modifying the ventilation system (e.g., installing local exhaust or relocating outdoor air intake away from the exhaust area)
• modifying operating procedures (e.g., turning off engines while waiting for service)
• removing or relocating the source
• inspecting for leaks in gas-fired heating systems and for backdraft-ing of combustion products.