IISection
4.3 InvestIgatIng Iaq pRoblems
When the indoor air problems grow into full-scale concerns, the owners and opera- tors run immediately to outside expertise. A lot can be done, however, through sim- ple in-house keeping before such moves become necessary. The IAQ investigations can be divided into five phases for either simple in-house inspection or complex diagnosis (Hensen and Burroughs 1998):
Phase I: Preliminary Assessment—a self–evaluation; data gathering; observa- tion effort.
Phase II: The Qualitative Walk-Through Inspection—conducted by trained in-house staff or as a preliminary inspection by the diagnostic team.
Phase III: Simple Qualitative Diagnostics—more extensive analytical proce- dures conducted by the diagnostic team; limited measurements of impli- cated factors or surrogates.
Phase IV: Complex Quantitative Diagnostics—broad in-depth testing; quali- tative studies of factors in combination; medical examinations.
Phase V: Proactive Monitoring and Recurrence Prevention—observation;
record keeping; retesting as warranted; preventive measures.
The preliminary assessment forms or questionnaires are developed and inter- views are taken for background evaluation, observational data gathering, and self- evaluation. In contrast to the preliminary investigation, the walk-through inspection involves a more thorough examination of the facility and the HVAC system by in-house personnel. In phase III, the walk-through inspection is conducted by expe- rienced and knowledgeable personnel. It usually involves the use of temperature,
humidity, and airflow measurement and monitoring. CO2 measurements are some- times taken to assess the effectiveness of the ventilation system. In phase IV, more precise sampling of various parameters, that is, ventilation parameters as well as pollutants, is conducted, which is further analyzed to evaluate the intensity of the problem and to take the preventive measures in phase V.
4.3.1 Sampling/monitoringand modelingof indoor air pollutant
Sampling techniques can be broadly classified as continuous, integrated, and grab or spot sampling. Continuous sampling provides real-time sampling and is used to observe temporal fluctuations in concentrations over short periods. Integrated sam- pling provides an average sampling over a specific period. It is used when the mean concentration is either desirable or adequate for the purpose. Grab sampling provides single samples taken at specified intervals. It typically consists of admitting an air sample into a previously evacuated vessel, drawing a sample into a deflated bag for later analysis, or drawing (by a mechanical pump) a sample through a sample
table 4.4
medical symptoms of sick buildings
syndrome symptoms
Sick building syndrome Lethargy and tiredness
• Type 1 Headache
Dry blocked nose Sore dry eyes Sore throat
Dry skin and/or skin rashes
Sick building syndrome Watering/itchy eyes and runny nose, that is, symptoms of an allergy such as hay fever
• Type 2
Humidifier fever Generalized malaise
• Flu-like symptoms Aches and pains
Cough Lethargy Headache
• Allergic reaction in sensitive individuals Chest tightness Difficulty in breathing Fever
Headache
Occupational asthma Wheeze
Chest tightness Difficulty in breathing
Sources: WHO. 1983. Indoor Air Pollutants: Exposure and Health Effects. WHO, Geneva;
Wilson, S. and Hedge, A. 1987. The Office Environment Survey: A Study of Building Sickness. Building Use Studies Ltd., London, UK.
collector to extract a contaminant from air; it is suitable when “spot” samples are adequate for the measurement of pollutants, and knowledge of temporal concentra- tion variation over short periods is not important. Different sampling techniques are available for various pollutants. Particulates are sampled in mass according to their sizes, and gravimetric analysis techniques are in use. The major techniques devel- oped for sampling gaseous pollutants are broadly classified as passive (based on membrane permeation or diffusion through a geometrically defined air space) and nonpassive (in which air pumping devices draw volumes through devices of known collection efficiency). Various sampling, measurement, and analysis techniques for air pollutants are listed in Table 4.5.
The concentration of a pollutant indoors depends on the relationship between the volume of air contained in the indoor space, the rate of production or release of the pollutant, the rate of removal of the pollutant from the air via reaction or settling, the rate of air exchange with the outside atmosphere, and the outdoor pollutant concen- tration (Maroni et al., 1995). The extent of indoor air pollution can be estimated with numerical models; mass balance equations are used to estimate the concentrations of indoor pollutants as fractions of outdoor concentrations and to estimate infiltration rates, indoor source strengths, pollutant decay rates, and mixing factors. In estimat- ing the total exposure of humans to pollutants (exposure to pollutants encountered indoors and outdoors, in industrials sites and other workplaces, etc.), it is essential to know not only the pollutant concentration, but also the individual patterns of mobility and use of time. However, the actual human exposures are often difficult to quantify.
This is largely because the behavior and activity patterns of individuals can strongly affect their levels of exposure (Harrison, 1997).
table 4.5
sampling, measurement, and analysis techniques
s. no. pollutant method
1. Carbon monoxide Nondispersive infrared (NDIR) photometry
2. Ozone Chemiluminescence
3. Nitrogen oxides Chemiluminescence
4. Sulfur dioxide West Gaeke colorimetric method
5. Formaldehyde Spectrophotometer analysis
6. Asbestos Phase contrast microscopy (PCM), scanning
electron microscopy (SEM), transmission electron microscopy (TEM)
7. Radon Thermo luminescent dosimeter (TLD)
8. Particulate matter (PM) Size selective samplers
9. Biological pollutants (pollens, etc.) Samplers with adhesive coating on slides
10. Ventilation rates Tracer gas technique
Sources: USEPA. 1991. Exposure Factors Handbook. EPA/600/8-89/043. U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Washington, DC (online publications); Persily, A. 1989. ASHRAE Journal, 31(6), 52–54.
The starting point in developing an IAQ model is usually a statement of the mass balance concerning the pollutant of interest (Figure 4.2). For example, consider a structure of volume V, in which makeup air enters from the outside and passes through the filter at a rate qo. Part of the building air is recirculated through another filter at a rate q1, and air infiltrates the structure at a rate q2. Each filter is character- ized by a factor F ≡ (Cinlet – Coutlet)/Cinlet. Usually the pollutant concentration is assumed to be uniform throughout the structure. The indoor and outdoor concentra- tions at time t are C and Co, respectively. The rate at which the pollutant is added to the indoor air owing to internal sources is S. The rate at which the pollutant is removed from the air owing to internal sinks is R (Wadden and Scheff, 1983). In this case, the appropriate starting equation is
= Input ratedue tomakeup,recirculatedand infiltratedairo o − o + 1 − 1 + 2 o− o+ +Output rate1 2 +Source rate −Sinkrate
dd (1 ) (1 ) ( ) .
V Ct q C F q C F q C Q q q C S R (4.1)
Or more simply, it can be written as
IN IN OUT OUT
d ,
d
V C C Q C Q S R
t = − + − (4.2)
Building Volume V Building surface area A Building source S Building sink R Indoor concentration C Outdoor concentration Co Infiltrated air
q2Co Filter intake fan qoCo (1 –Fo)
Filter q1C(1 –F1) Makeup air
qoCo
q3C Exfiltrated air
Exhaust fan
Exhaust air
Recirculated air q4C
fIguRe 4.2 IAQ mass balance for an HVAC building. (Adapted from Shair, F.H. and Heitner, K.L. 1974. Environmental Science and Technology, 8, 444; Wadden, R.A. and Scheff, P.A.
1983. Indoor Air Pollution: Characterization, Prediction, and Control. Wiley, New York.)
where V is the volume of the room; C is the pollutant concentration in the room; CIN is the concentration entering the room; QIN is the airflow into the room; COUT is the concentration leaving the room; QOUT is the airflow leaving the room; S is the source term; and R is the removal term, which includes pollutant removal by using air cleaners and sinks.
From the well-mixed assumption, COUT equals C. The equation can be rewrit- ten as
IN IN OUT
dd .
V Ct =C Q −CQ + −S R (4.3)
This is the case with single compartment mass balance modeling (NRC, 1981).
It is applied to the spaces that are considered as well mixed at a given point of time and the pollutant concentration remains the same at all locations within the volume being modeled. Despite the widespread use of the well-mixed assumption, there is evidence that in many cases, indoor air concentrations are not spatially homoge- neous. Well-mixed models underestimate concentration near an emission source.
Recently developed models consider the “source proximate effect” (Furtaw et al., 1996), which is based on the premise that concentrations in proximity to the source are higher than those predicted by well-mixed models. Multicompartmental model- ing is the approach to modeling nonhomogeneous indoor air concentrations. This approach divides the space into two or more well-mixed zones that are connected by interzonal airflows (Furtaw et al., 1996). A compartment is defined as a region within which spatial variations in pollutant concentrations can be neglected over the timescale of interest. Depending on the ventilation conditions, a single room, a floor, or a whole building may be adequately approximated as a single compart- ment. However, if either sources or sinks are not uniformly distributed throughout the region of interest and the rate of mixing throughout the region of interest is low compared with characteristic residence time, then the single-compartment model may not provide an adequate description.