Airborne filter measurements

A summary of airborne radioactivity measurements in Monthldry, France is presented in the report by Milles-Lacroix et al. (1994). Radioactive aerosols particles are collected in surface air, using cellulose filters and high output pumps (110 m%). Equipment operates on a permanent, 24-hour a day basis, and filters are replaced daily throughout the year. The volume of filtered air is 2600 m per day. After removal, individual filters are subjected to alpha counting, using a ZnS counter, and to beta... 

The OSHA limits, regulations, and recommendations apply to in-plant air quaUty. Improperly filtered exhaust air may cause a plant to be in violation of the EPA standard, therefore these data should not be confused with the EPA limit for airborne lead, 1.5 fig lead/m, measured over a calendar quarter, which pertains to the exterior plant environment and emissions. The installation and proper maintenance of exhaust filtration systems enables most plants to comply with the EPA limits for airborne lead (see Lead compounds, industrial toxicology). 

Because a filter sample includes particles both larger and smaller than those retained in the human respiratory system (see Chapter 7, Section III), other types of samplers are used which allow measurement of the size ranges of particles retained in the respiratory system. Some of these are called dichotomous samplers because they allow separate measurement of the respirable and nonrespirable fractions of the total. Size-selective samplers rely on impactors, miniature cyclones, and other means. The United States has selected the size fraction below an aerodynamic diameter of 10 /xm (PMiq) for compliance with the air quality standard for airborne particulate matter. 

Particulate filtration removes suspended liquid or solid materials whose size, shape, and mass allow them to remain airborne at the air velocity conditions present. Filters are available in a range of efficiencies, with higher efficiency indicating removal of a greater proportion of particles and of smaller particles. Moving to medium efficiency pleated filters is advisable to improve lAQ and increase protection for equipment. However, the higher the efficiency of the filter, the more it will increase the pressure drop within the air distribution system and reduce total airflow (unless other adjustments are made to compensate). It is important to select an appropriate filter for the specific application and to make sure that the HVAC system will continue to perform as designed. Filters are rated by different standards which measure different aspects of performance. 

It has been demonstrated that it is possible to lower the level of airborne radon progeny by filtering and/or expose the air to an electric field. If the radiological risk is measured by the potential alpha energy concentration (PAEC) or exposure rate, for instance expressed in J m 3 or WL, the level may be lowered to about 10-20 % of the value in untreated air, while the reduced level may only be about 40-50 % of the untreated one if the average dose to a certain part of the respiratory tract, for instance expressed in Gy year 1, is considered. 

HEPA filter evaluation (airborne particulate levels, smoke-challenge testing, velocity measurements, etc.)... 

Airborne particles were collected outside the laboratory by drawing air at the rate of 30 cu. meters/day through a 4.7 cm. diameter, 0.8-p pore size, membrane filter (I). The air intake was 5 meters above ground and was shielded from precipitation by a glass tube. Air was drawn through the filter for 24-hour periods. The filter was collected at 8 30 a.m. (either EST or EDT, as applicable), Tuesdays through Fridays, and the a-activity was measured with a scintillation detector within 2 minutes of collection. 

Modified filter sampling methods that are available will measure ambient levels of strong acid in ambient aerosol samples, and these methods do so with acceptable precision and accuracy [as indicated by the balance between measured anions and cations (56, 57)] in the absence of significant levels of particulate weak acids. Additional intercomparisons involving intrinsically different techniques for particulate strong acidity [e.g., IR spectroscopy (48), thermal speciation (38, 45), and filter methods (28)] are needed. Further information on the occurrence of various weak acids in airborne particles is needed, along with further studies of techniques for their specific determination in atmospheric aerosol samples. 

Table 3.2 shows measurements of particulate and gaseous stable iodine in the atmosphere. Moyers et al. (1971) used membrane filters and activated charcoal to collect the particulate and gaseous fractions in air at Boston, and found the ratio Ip/Ig to be correlated with the concentration of particulate lead in the air. It was not inferred that Ip was combined with particles of lead, but rather that the concentrations of lead served as an index of the total airborne particulate. Moyers et al. expressed their results as... 

Step 7b. For detection of airborne radon progeny. Filter a reagent blank solution that contains the BaS04 on the tared filter paper. Wash the precipitate that remains in the beaker to the filter by rinsing the beaker with a jet of deionized water. Wash and dry the precipitate on the filter with 10 mL of ethanol, followed by 10 mL of diethyl ether. Draw air through the sample for a measured time of 30 minutes to 1 hour to collect airborne radon and... 

XRF is the simplest of these methods. It allows bulk analysis of solid or liquid samples with detection limits of approximately 0.1 pg. The method can thus only compete with radiometric methods for the longest lived of radionuclides. It has approximately the same sensitivity for Th as alpha spectrometry but has the advantage that little sample preparation is required and that analysis is rapid and easily automated. XRF would be the method of choice for measurement of airborne thorium collected onto filter papers, for example. 

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