Health Effects of Atmospheric Particulates
Types of particles
Effects of atmospheric particulate matter
Size and setting rate of atmospheric particles
Control of particles
Aerosols: Solid or liquid particles smaller than 100 x 10-6m (100 microns).
Condensation Aerosol: Aerosol formed by condensations of vapor or reactions of gases.
Dispersion Aerosol: Formed by grinding solids, dispersion of dust, atomization of liquids.
Fog: High level of water droplets.
Haze: Decreased visibility due to presence of particles.
Mist: Liquid particles.
Smoke: Particles resulting from incomplete combustion function.
The smallest aerosol would be only two or more molecules, ammonium sulfate would be an example. At the other extreme would be a sulfuric acid particle at 30% relative humidity. This aerosol would consist of about 10,000 molecules of H2SO4 and have a diameter of 0.01 m m (1 x 10-8 m)
Particulate matter enters the atmosphere from many different sources. Wind blown dust is the most obvious source of particulate matter with which we are familiar. Wind blowing over the oceans picks up salt particles, and all combustion processes release both solid matter (soot, smoke, fly ash) and gases which react to form larger molecules that can be classified as aerosols or particulate matter.
To a first-order approximation, the chemical compositions of an aerosol governs its toxicity, and size the particles physical properties. Its density and diameter determine the most important aspects of an aerosols behavior in the atmosphere. Aerosol diameter is a descriptive term, because most particulates are non-spherical and irregularly shaped.
Figure 34.1 Relation of diameter to properties of particulate matter air pollution
Stokes Law is the relationship that relates the "settling rate" to a particle's density and diameter. Stokes law is given as:
u = settling velocity in
g = acceleration due to gravity in
d2 = diameter of particle squared in
r 1 = particle density in
r 2 = air density in
h = air viscosity in poise
What Stokes Law tells us is that, all other things being constant, dense particles settle faster, larger particles settle faster, and denser, more viscous air causes particles to settle slower.
Stokes Law is used in several ways. We can predict settling rate for a given particle if its diameter and density are known. Another way in which Stokes Law is used is to estimate particle diameters (called "Stokes diameter") from observed settling rates.
What is Stokes diameter of a particle which falls at 1 in (0o C, 1atm),
air . . .
d = 0.0177 cm which equals 177 m m.
This is a very large particle (perhaps it could be wind blown dust), but the settling velocity we used for this example was quite high.
When air is sampled for particulate matter, particles are collected according to the effective or Stokes diameter. Particulate matter is classified as "Total Suspended Particulate", TSP, PM-10 and PM-2.5. TSP is the total mass of particulate matter suspended in air, and the units of this measurement are m g/m3. PM-10 refers to the total mass of particulates in an air sample that are smaller than 10 m m in diameter, and likewise, PM-2.5 is the mass of particulates in an air sample that are less than 2.5 m m m in diameter.
Monitors sort particles according to mass (which is proportional to diameter for ideal spheres). Since particulate matter varies widely in its composition and size, an average value is used to describe experimentally collected aerosols. The mass median diameter (MMD) is used to describe experimentally collected particulate samples.
Samplers are calibrated using "ideal" spheres of known diameter, density and mass. The calibration plots from the standard particulates are then used to determine the percentage of mass less than a specified diameter of actual air samples. The MMD plot consists of a graph where y-axis is log scale of diameters; x-axis is percent of particles with mass less than this diameter.
Particulates affect health by transporting toxic substances into the respiratory tract. Toxic compounds (lead, cadmium, beryllium, PAHs, etc) can then be adsorbed into the body where they affect biological processes as if they had been ingested by any other means. If the particles
Figure 34.2 shows the efficiency with which particulate matter is retained in the respiratory tract for mouth breathing. The point to note here is that larger particles are deposited in the respiratory tract with high efficiency, particles between 0.5 and 0.1 m m are retained with efficiencies of about 10%, and then the smaller particles are retained with greater efficiency. This data is interpreted to mean that the large particles are removed in the mouth and throat, while the very small particles (<0.10 m m) are taken deep into the lungs where they impact in the many tiny air pockets of the soft lung tissue. The particles that are not retained are small enough to navigate the air passages of the mouth and throat, and move slowly enough that they are expelled when a person exhales. Unfortunately, the tiny particles that are retained deep in the lungs contain the highest concentrations of toxic substances.
Figure 34.2 Respiratory tract deposition efficiency for aerosols resulting from mouth breathing.
Figure 34.3 Respiratory tract deposition efficiency for aerosols resulting from nose breathing.
Original page created by Dr.Richard Foust, Professor of Chemistry and Environmental Science, Northern Arizona University