> Silicosis is only caused by breathing crystalline silica particles in
> the respirable size range, which is less than 0.2 microns. Beach sand
> does not contain such particles (not that I'm suggesting you should
> breathe beach sand).
> Same problem, essentially none of the dangerous particles are in the
> air over a dirt road. Breathing the dust from a dirt road is not
> especially good for you, but will not cause silicosis.
> Silicosis is a "cumulative exposure" disease ... every particle that
> finds it's way into your lungs will reduce your lung capacity by some
> amount. Long term exposure to even tiny amounts can have a severe
> cumulative effect.
No sir, you are mixing a partial understanding of respirable. It has to
do with how deeply into the lungs the particles must go in order to do
damage. You have three universally recognized categories, inhalable,
thoracic and respirable. Inhalable are those that do damage anywhere
from the tip of the nose to the deep lung. Thoracic do damage from the
brachial tubes and deeper. Respirable do damage only when down deep in
the gas exchange region of the lungs.
You are mistaken with regards to silicosis and the belief that every
particle stays and does damage. This is what the respirable fraction
ratio to size is about. Large particles cannot enter the deep tissues
in the first place due to physical size. As the particles get smaller,
they are more likely to be able to enter the deep tissues of the lung.
Hence the slope of the line regarding fractional retention. Further, of
those particles that do enter deep lung tissue, not all are retained.
Many are exhaled. This to is part of the function of the slope of the
Please do see Appendix C: Particle Size-Selection Sampling Criteria for
Airborne Particulate Matter, of the current 2004 ACGIH TLV and BEI's
handbook. It's pages 73-76.
Dirt roads are one of the primary sources of PM2.5 or PMfine as it
often called. Because of the grinding action of the vehicles, such
small particulates are generated. Same with the beaches, though to a
lesser degree. Beaches depend on the type of sand, type of water, etc.
A calm coarse sand will have less PMfine then a heavily trafficked fine
sand beach will have. This has been well documented by the US EPA, and
others, who have performed studies on the subject.
PMfine can be controlled. Not all control devices are equivalent to
chickenwire. HEPA filtration, electrostatic precipitators, etc all
achieve various measures of control of fine particulate matter.
Effectiveness varies, and is typically directly related to the cost of
the control device. Furthermore, a closed loop system has no emissions.
In the case of sand blasting, closed loop systems are easily utilized.
For information on control devices and PM2.5, I'd suggest searching the
EPA TTN web site.
The effectiveness of dilution in reducing exposure to toxic materials
is well known and documented. You can obtain supporting background
documentation from the EPA Scram web site containing air pollution
modeling software and guidance documents. You can also read NIH and
NIOSH documentation of toxic exposures and the effect of dilution. It
is the fundamental reason why stepping back from a camp fire has you
breathing less smoke and coughing less. Same with any air pollution
operation. Distance creates dilution, dilution reduces exposure,
reduced exposure reduces risk.
Air Toxics Section Head
Maryland Department of the Environment