Horses were not only used to work in the mines – underground as “pit ponies,” but also to deliver the coal to houses, for heating and cooking during the 19th and early part of the 20th century.  By the 1940’s Electricity and Gas had become more commonplace, even in the rural areas. 
Having grown up in the country - Suffolk (East Anglia), one of my school friends’ father was the local coalman.  I recall that he and his assistant were always covered in the coal dust, head to foot and they had special garments that were made of leather to covered their backs while lugging the coal into the customers “coal scuttles.”   
By the time I was in my early teens, the horses had been replaced by the mighty “lorry,” or “truck.”  The the horses were “put out to pasture,” which of course meant that they were now pets that were generally ridden by the children of the coal merchants or the upper class residents of the area.
Growing up in our English country cottage where the only heat was a coal fire we were glad of the local “coalman.”  Generally, he was well respected in the village community, for without him and his trade, the only alternative materials to burn was wood, from the lust local “black forest.”

With the passing of the Mines & Collieries Act in 1842 (Miners Act 1842), English Parliament, which put an end to women, girls & boy under the age of 10 working in the underground mines, pit ponies were “harnessed” in to service.
Approximately 45 years later in 1887 the Coal Mines Regulation Act issued a “national” protection for horses working in the underground mines.
Of course England wasn’t the only country using “horse” power to mine the much needed coal to heat homes, cook meals and run the industrialized manufacturing.
Germany, Australia & Nova Scotia, to name but a few also used “Pit Ponies;” however, in America, mules were more plentiful than ponies, so were found in the coal mines in far greater numbers than the ponies.
According to the British National Coal Board, only 55 ponies were still in service in 1984, chiefly in the Northumberland area – Ellington Pit, down from 70,000 (yes, seventy thousand) in 1913.  One of the last surviving ponies was called “Tony” who was reported to have died in 2011 at the grand ole age of 40, but “Robbie” who was thought to be the last colliery horse to work underground, retired in May 1999! 
When the Centerville Pit, Iowa, closed in 1971, it was reported to be the last mine in the USA to have used pit ponies, whereas “Wharrier,” & “Mr. Ed,” of the Collinsville Coal’s No 2 Mine, Australia retired in 1990.  

These photos show the PM10 & PM2.5 size silica particles – seen in the sunlight beam, but they are still present even when they are not “highlighted” by sunlight.  Where is this Silica?  It’s in the footing of your arena.
“The definition of Crystalline Silica particles, size PM10 & PM2.5:
Crystalline silica is the basic component of soil, sand, granite and may other minerals.  Quartz is the most common form of crystalline silica.
Crystalline silica has been classified as a human lung carcinogen.  Additionally, breathing crystalline silica dust can cause silicosis, which in severe cases can be disabling, or even fatal.  The respirable silica dust enters the lungs and causes the formation of scar tissue, thus reducing the lung’s ability to take in oxygen. 1  (A leading contributor to Pneumonia, Bronchitis and COPD)
PM10 & PM2.5 particle size can remain airborne for up to 24 hours and when inhaled these particles lodge deep into lung tissue and are a true health hazard…the public is generally not aware of the sever health effects  of dust.”
1 OSHA:  “Crystalline Silica Exposure” Health Hazard Information for Construction Employees.  http://www.osha.gov/Publications/osha3177.html.

Horses breathe up to 75 liters of oxygen per minute (4500 liters per hour). 
If the air is filled with dust and the horses breathe it, how does it affect them? 
However, unlike filters the horses lungs can NOT be cleaned or replaced and the long-term exposure to dust hazard creates irreversible damage resulting in health problems.
Long term exposure to silica dust is related to Chronic Obstructive Pulmonary Disease (COPA) also known as “heaves” similar to asthma in humans.
Long term exposure to silica dust builds up in the lung tissue and causes fibrous scar tissue to form on the lung walls creating difficulty for adequate oxygen supply during exercise.
 
This adds pressure to the horse’s capillaries when the horse breaths deeper and harder than normal during exercise.
This forces the oxygen into the air sacs thereby creating a negative pressure which may cause rupturing of the small capillaries also called Exercise Induced Pulmonary Hemorrhage (EIPH), or “bleeders.”
Not a great environment for either horse or rider!