Factors Influencing the Dose of Aerosols: Deposition and Calculations, Slides of Public Health

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FACTORS INFLUENCING THE DOSE FORAEROSOLS

A.

FACTORS INFLUENCING TOTAL DOSE

a)^

Concentration in air,

C , (mg/m

3 ) or (mg/L) or

(mM/L) or fibers/cc or mppcf or any appropriate unit ofmass or number of particles but

not ppm.

b)^

Particle size, which will determine the fraction of aerosol deposited. This fraction is called

α^ and is given in

%.c)^

Minute ventilation, MV, which is determined by tidal volume (

VT

) and number of breaths per minute (

f ), i.e.,

VT

^ f

d)^

Duration of exposure in minutes (

t ).

e)^

Body weight (

kg ) or body surface area (m

2 ) or lung

surface area (m

2 ^

70m

2 in adult humans).

To get a “ball park” figure, assume 50%(0.5) for

α^ if particle size is not known.

Thus, if a human is exposed at aconcentration of 1mg/L for a period of60 minutes and has a

VT

of 800 ml and

f^ of 15/minute the total dose receivedwould be:

This value can then be divided by body weight, etc.for a true expression of dose.As can be seen, we now have the proper units fortoxicity instead of just having an exposureconcentration in mg/L or mg/m

3 or ppm.

Comparisons of total dose received can be madebetween various animal species exposed at the sameconcentration, on the basis of mg/kg of body weightor mg/m

2 of body surface area or pulmonary surface

area, or for surface area of any portion of therespiratory tract.

B.

FACTORS AND MECHANISMS AFFECTING TOTAL DEPOSITION AND REGIONAL DEPOSITION OF AEROSOLSThe factors affecting deposition sites in the respiratory

tract are:

SizeShapeDensityThe above 3 characteristics are taken into accountsimultaneously for particles > 0.

μm by experimental

determination of their aerodynamic mass equivalentdiameter and for particles <0.

μm only the size is

important, as presented above.

• Electrostatic attraction (depends on chemicalcomposition) between the particle and thesurface of the respiratory tract, may play a rolebut is difficult to investigate.- Hygroscopicity (depends on chemicalcomposition) since a small particle can growupon entering the 100% RH of the respiratorytract. Also we must take into account the following factor:

Pattern of pulmonary ventilation including nose vs. mouth breathing, as well as volume andfrequency of each breath.

1.^

Impaction, Inertia When an obstacle exists in the path of theairflow, or bifurcations or tortuous paths occursuch as in the nose or tracheobronchial tree,small particles will follow the air flow linesbut large particles, because of greater inertiaare unable to change direction and willimpact. Note: Impaction is the most important mechanism ofdeposition

for particle > 3

μm in the

nasopharyngeal area and central airways of thelung.

2.^ Sedimentation,

Settling

All particles with density greaterthan air experience a downwardforce due to gravity. Thus, a particle accelerates downward until itsvelocity increases to the point where the retardingforce due to its motion through air just balances itsweight. If a particle is spheric and small enoughso that viscous forces are the primary resistiveforces, Stoke’s law applies to predict retardingforces.

4.^

Electrostatic Attraction

All aerosol particles have a + (non-metallic) or - (metallic)charge which may affect deposition. However, little isknown about the effect of charge except for highly chargedparticles which can occur in freshly generated particles.Important for polymeric fibers, proteins (synthetic ornatural) which can hold a high charge.

5.^

Interception

Of importance for fibers as the inspired air comesin close contact with a surface. Fiber deposition modelsare less well developed than for particles.

C.

DEPOSITION CALCULATIONS AND RESULTS

From the above, calculations of deposition according toaerodynamic particle size have been made for thevarious regions of the respiratory tract:

a) Nasal-Pharyngeal (N-P)b) Tracheo-Bronchial (T-B)c) Pulmonary (P)

b)^

The smaller the animal, the larger the probability of nasal retention.

A 2

μm particle has about the same

probability of being retained in the mouse nose as an 8 μm particle in man. Important for large particles.c)^

The larger the particles are above 3

μm the more

chance of difference in regional deposition with humans.Total net dose may be the same but biological effects maybe different because the deposition sites and clearancerates are different.(5) In particular, clearance rates varywidely between different species.(6) In general,clearance in rats and mice is faster than in humans,monkeys or guinea pigs.

d)^

To have the same relation with humans between

atmospheric particulate concentration and rate ofdeposition of particles in the

lungs, fairly uniform

particles around 1-

μm (MMD) seems to be highest to

use with small rodents, this would be defined as“inhalable” or “inspirable” and would also beclassified as “respirable” particles for small laboratoryanimals. See below for the descriptions and definitionsof these terms.