Physical Diameter versus AMAD for Particulates

DOE-HDBK-1184-2004

Deposition fractions affect clearance rates of particulates from the lung to the

gastrointestinal (GI) tract. Since urine excretion of tritium dissolved from tritiated

particulate over time is a function of those particles which have not cleared from the

lung, interpretation of urine excretion curves can extract particle size information.

This requires extended monitoring without additional tritium intakes, which can

adversely affect project management activities. To estimate intakes (and therefore

doses) accurately and promptly from urine data, detailed knowledge of the

particulate dissolution rates must be available. Tritiated material and particle size

distribution involved in the intake, and the expected dissolution rate for that material

and size, must be known. When tritiated materials and sizes are likely to range

broadly, it will be difficult to provide material identification and size characterizations

in the workplace. Moreover, although dissolution rate data are available for several

materials, these data do not include all the possible combinations of materials and

particle sizes that might be encountered.

5.2.2.2 Physical Diameter versus AMAD for Particulates

The ICRP and most technical papers that address respirability of particulate

contamination use the term "activity median aerodynamic diameter" or "AMAD"

when referring to particle size distributions. The AMAD can be very different from

the physical diameter of a median particle, as it is a function of the particle's density

and shape. The relative deposition of particles in the respiratory tract is dependent

on the AMAD. Particles with small AMADs tend to deposit deeper (higher

percentage in the alveolar interstitial region) in the lung than larger AMAD particles;

because of the slow clearance rates of the deep lung, smaller particles generally

cause higher lung doses than larger ones. For particles with AMADs greater than

about 10 m, the fractional deposition in the deep respiratory region is considered

negligible. The larger sized particles tend to be deposited in the upper respiratory

region where they are rapidly removed to the gut and eliminated in feces. The lung

dose per unit intake is less for larger AMAD particles than for smaller AMAD

particles. Table 5-6 shows a comparison of physical diameter versus AMAD

(calculated per Cheng 1999a) for a variety of materials from low density to high

density.

Table 5-6. Physical diameter versus AMAD for monodisperse particle distribution

(s g = 1)

Physical Diameter (m) vs. AMAD (m)

Base Material* AMAD = 10 AMAD = 5

AMAD = 2 AMAD = 1

AMAD = 0.5

Organic[∼(CH2 )n]

12.9

6.48

2.61

1.31

0.668

Rust[∼FeO(OH)]

5.87

2.92

1.15

0.559

0.265

Ti H2

6.17

3.07

1.21

0.591

0.282

Zr H2

4.76

2.36

0.920

0.441

0.202

Hf H2

3.53

1.74

0.666

0.310

0.133

* Variable amounts of elemental hydrogen are isotopically tritium.