Breathing Rates and Tidal Volumes

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

conservative estimate, as laid out in the ICRP -30 methodology. Particles of this size are likely to result in

the greatest deposition in the pulmonary region of the lungs. The actual size distribution can be measured

with instruments such as cascade impactors, but these are not practical for continuous operation in the

work-place environment. Electronic instruments can give continuous information about the optical particle

size, but not the AMAD. Thus, particle size can only occasionally be measured to typify the size

distribution in a particular situation.

Size-selective inlets for air samplers have been developed to mimic deposition in the respiratory tract,

giving more accurate estimates of deposition in the pulmonary region. Non-respirable or noninhalable

particles are removed by the inlet, and the respirable or inhalable fraction is collected on a filter. These

devices can be useful in minimizing the dose assessment errors resulting from uncertainties regarding the

actual aerosol-size distribution; however, they require additional handling and care, and require separate

samplers for total airborne activity. If the AMAD is often substantially greater than 1 m in an area, the

addition of samplers with size-selective inlets may be worthwhile. Regulations allowing the substitution of

size-selective samplers are not established, however, so special arrangements may be needed with

regulatory agencies.

Breathing Rates and Tidal Volumes

The actual air intake of a worker can vary from 5 L min-1 to 100 L min-1, although typical variations

from the assumed 20 L min-1 standard will probably be no larger than a factor of 3. Total air intake depends

on the rate of breathing and on the volume of tidal air. The velociy of this air influences the regional

deposition of aerosol particles. Newer, more sophisticated lung models include this breathing-rate effect in

calculation of dose distribution. Information about individual breathing behavior may be useful in the

application of the newer lung dosimetry models. Simpler models, such as ICRP-30, assume that regional

deposition is independent of breathing rate, with total deposition determined only by the volume breathed.

Particle Solubility and Lung Clearance

When partic les are deposited in the respiratory tract, they are cleared from airway surfaces by several

mechanisms. Insoluble particles are cleared by the biomechanical means of macrophage and mucociliary

transport, while some particles are retained in pulmonary tissues. Particles of soluble material dissolve,

making the contaminant available for other means of transport such as absorption into the blood.

Dosimetry of the contaminant depends on how fast the particles dissolve.

Rate of particle dissolution is divided into three categories by the ICRP-30 model. Classes D (days),

W (weeks), and Y (years) refer to the retention time of the material in the pulmonary region of the lungs.

A retention half-time of less than 10 days is retention class D, a half-time of 10 to 100 days is class W, and

half-time greater than 100 days is class Y. Some materials have been described to have characteristic rates

of dissolution and are associated with a particular retention class. Many factors can affect the dissolution

rate, however, so general assignments to retention classes should be regarded with caution.

The health physicist may have some prior knowledge of the chemical compounds of the nuclides

present in an area and may be able to assign them to retention classes. The ICRP -60 dosimetry model

provides for a lung retention class designation of aerosols depending on the rate of dissolution; however,

actual determination of the lung class for dose assessment can best be determined after an exposure

utilizing appropriate chemical and bioassay data, but this can only be accomplished in retrospect. A

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