Grounds for Assessing TLs for Industrial Air Quality
In order to assess the target concentrations of air contaminants both human risk-based and technology-based approaches can be used (Fig. 6.7). The various approaches are dealt with in more detail elsewhere.3’4
The procedure of assessing health-based occupational exposure limits for chemical substances includes determination of no-observed-adverse-effect level (NOAEL) for the critical toxic effect and application of an appropriate safety factor based on expert judgment (see Section 5.3). In principle, the same procedure could be used for assessing the TLs. However, the quantitative risk assessment procedure entails notable uncertainties at low-dose regions—say, below one-tenth of the current OELs. In addition, exposure limits are revised at certain intervals in the light of new research information and actual policy objectives. In most cases, the limits have been reduced over the years. In theory, one possibility for assessing a target level for desired air quality could be the determination of an exposure that cannot be distinguished from the biological monitoring values of the nonoccupational population. However, ade quate data for this purpose exist only for a few substances in advanced industrialized countries, and for that reason a technology-based approach for target level assessment is considered in this paper. Similar control strategies, based on performance standards and risk assessment, have been proposed for some industries—for example, the pharmaceutical industry and technology transition in the defense sector.5’6
In the technological approach, qualitative and quantitative information on emissions released by various production and work processes, as well as data on control technology performance, are required in order to specify the air quality target levels that are technically and economically feasible. The approach is based on information on current concentration levels that are achieved by different control technologies, ranging from standard practices to the most advanced technology options (Fig. 6.7).
Existing contaminant exposure data banks can be utilized to survey the standard practices.7’8 For example, the register of occupational hygiene measurements at the Finnish Institute of Occupational Health, containing information on more than 150 000 determinations of airborne pollutants, has been used. In most cases the measurements have been requested due to concern over working conditions. Consequently, the data in the registry likely reflect worst-case circumstances, in which the exposure levels have been high due to heavy emission rates or inefficient control. Based on the data in the register, cumulative frequency distributions of contaminant concentrations were created. As an example, the cumulative frequency distribution of concentrations of xylenes is shown in (Fig. 6.8). The median concentration levels and the lower and upper tails can be seen from the cumulative distribution curves. Usually attention has been focused on higher concentrations in order to prioritize the required control actions. However, from a design point of view, low values are of particular interest, as they reflect conditions in the work rooms with efficient engineering controls.
Target levels of indoor air quality
Air quality with standard technology
Air quality with advanced technology
Health and comfort effects
FIGURE 6.7 Approaches for the assessment of target level air quality.
In recent years, benchmarking has proved to be a very successful tool in total quality management (TQM).9 Basically, benchmarking is a targetsetting and comparison process in which the current standard performance is compared with the best possible performance. A typical feature of the benchmarking process is periodic upgrading of the targets. Applying the benchmark philosophy to air quality control means that the air quality level produced by the best available technology (BAT) must be defined. In this section, the benchmark air quality is obtained by determining the contaminant concentrations in plants with advanced production and control technology. For selection of the benchmark plants, the following criteria were set:
Concentration of xylene (mg/m3)
FIGURE 6.8 Plant maximum concentrations of xylene (number of plants measurements = 865).
• Effective elimination of emission sources through the selection of the best process technology or effective control of emissions from sources that cannot be avoided
• Balanced mechanical supply and exhaust ventilation equipped with an advanced air distribution strategy to accurately control the flow patterns in a work space
• Air-handling units equipped with heat recovery and sophisticated control of the key parameters of HVAC systems, such as temperature, airflow rate, and pressure difference
• New or renovated premises
As an example, the measurement data on inorganic total dust collected in con ventional factories and benchmark factories is shown in Fig. 6.9.
It is practical to present target concentrations in terms of concentration bands representing different categories of air quality (Fig. 6.10). The first category, representing the cleanest air, reflects the special requirements of certain processes (the electronics industry, biotechnology, etc.). The second category represents good occupational levels achieved by using BAT controls. For most contaminants the upper concentration limit of this category is less than one — tenth of the OEL for the corresponding compound, implying that the employer need not carry out repeated air monitoring according to the European standard.10 The next two categories for occupied spaces with notable contaminant sources cover the concentration range up to the OEL for a particular
. Median 1 L
FIGURE 6.9 Concentrations of inorganic total dust in the breathing zone in conventional and benchmark factories.
FIGURE 6.10 Target level categories of air quality.
Contaminant. In the following four quality categories, numerical values of target levels are given. It may be useful to set a fifth category, for nonoccupied zones or spaces—for instance, the upper part of a room. The occupied zone is defined in terms of certain distances from the floor and wTalls by a European standard.11
Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK