Last week I started a series about water with “Water Part I: Is God a Mathematician”.  I started working on Part II, but find I am too saddened by the loss of 59 lives in a dust explosion in an iPad factory in China to write about anything else this week.  If you are not familiar with what happened, here is a link to an NPR article about the tragedy.  Since dust is so familiar, we tend not to see it for the hazard that it can be.  
Before talking about dust explosions, let me run through a quick review of oxidation, fires and explosions.  Oxidation is a heat-producing chemical reaction between a material and oxygen.  Oxidation can occur very slowly, as in the case of iron developing a coating of iron oxide (rust), or faster, in the case of a campfire.  Oxidization releases heat which warms up surrounding objects.  Since oxidation proceeds more quickly at higher temperatures the process is self-reinforcing.  As the oxidation process speeds up, eventually it becomes hot enough to cause the nearby air to emit light, which we call fire.  Sometime oxidation occurs fast enough such that it heats the air around to an extremely high temperature.  This causes a rapid expansion of the air, creating a pressure wave which we call an explosion.
What makes dusts dangerous can be explained in terms of surface area.  Let’s go back to the example of a campfire.  If you take a match and hold it next to a large log, you can oxidize a small section of the surface of the log, but it is nearly impossible to get a self-sustaining fire.  If you take the same match and hold it next to a pile of small twigs you can easily light a fire.  The difference is due to the increased surface area per unit mass of wood for the pile of twigs.   As the first twig is being burned by the match it releases heat which warms the neighboring twigs making them more amenable to oxidation and allowing the fire to propagate. 
Dust is like the pile of twigs taken to the extreme with a very, very high surface area per unit of mass of the material.  Items that we don’t think of as flammable can burn explosively when ground up into a dust.  The explosion at the iPad factory in China was from aluminum dust.  You can take an aluminum Coke can and hold a match under it all day and nothing much will happen.  But if you grind the can into a dust, suspend the dust in the air and provide a source of ignition, you can generate an explosion.
Lest we think that these sorts of tragedies are limited to China, according to OSHA, the United States has experienced an average of 15 workplace dust explosions per year from 1980 to 2010.  The four most common materials involved in these dust explosions are coal, grain, wood, and aluminum.  You may remember the dust explosion at the West Pharmaceutical plant in Kinston, NC in 2003 that killed six workers.  The Kinston blast came from the dust of plastic (polyethylene). 
The hazards of dusts are generally known to the people who design manufacturing environments and ventilation systems.  (As a quick aside, the author of the NPR article that I linked above notes that the plant in China where the explosion occurred last week had sealed windows, implying that this was part of the problem.  In fact, you must have sealed windows in a dusty environment in order to properly and safely manage ventilation.)  Generally the ventilation systems are designed adequately to manage normal operations.  In most, but not all, accidents, there is some non-routine action which is being taken which contributes to the accident. 
Apple and its suppliers have had three dust explosions in iPad plants in China during the last year.  From the media accounts it would seem that the ventilation systems were adequate for safety in day-to-day operations, though just barely as the workers note the ability to see dust in the air and coatings of dust on surfaces.  Even the best ventilation system will capture slightly less than 100% of the dust.  Therefore, you must shut down operations on occasion to clean.  In the explosions in China last week and in December, the explosions occurred just after the workers had cleaned the plant in preparation for an inspection.  My suspicion is that some aspect of the maintenance activity set in motion the chain of events which led to the explosion. 
Whenever I hear of an industrial accident, like the one in China last week, I am especially saddened because I know that most of them could have been avoided.  The correlation between non-routine and/or maintenance activity and industrial accidents is very strong.  Generally speaking safety systems are designed for normal operations and some, but not all, maintenance activities.  This holds true for nearly all industrial operations, not just dust control.  If you or anyone you know works in a potentially dangerous environment, it is extremely important that safety reviews be conducted before any non-routine tasks.  It could save lives.
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