Any amount of DUST is dangerous – that is why OSHA enforces workplace dust hazard laws.  Environmental laws to prevent fugitive dust hazards have been used and enforced for many years.
The Centers for Disease Control (CDEC), The World Health Organization (WHO), The American Lung Association (ALA), The Occupational Safety & Heath Administration (OSHA), The Environmental Protection Agency (EPA), & the United States Department of Agriculture (USDA), all confirm and warn the general public about the dangers of PM10 & PM2.5 Crystalline Silica.
For details on the definition of Crystalline Silica PM10 & PM2.5 Dust, see the “Dust is Hazardous – period” (Link).
There are several methods of dust control – most are less effective than ArenaKleen.
Watering:  Easy, yes!  Cost, depending on if you are using a “well” located on the property or not.  How deep is the well in conjunction to the water table in your area and are the water tables at capacity due to weather.  Is your area experiencing a “drought” or not?  Being honest, most of us do not consider that possibility, but it bears consideration and will considerable affect the cost of watering.
The watering method:  Water surrounds the sand particles, which creates loose footing until the water has evaporated.  During the winter months the water can freeze, which makes the footing hard – do you want to be exercising your horses on a frozen surface?
Calcium & Magnesium Chlorides:  These are “industrial salts,” which “draws” moisture from the air in order to keep the dirt/sand/footings moist.  All well and good, unless you live and ride in an “arid” climate, like in the Southwest.
Points to consider with these Chlorides, they are very “hazardous,” even more so than the Crystalline Silica, when inhaled.  They are easily washed away during a Rain Storm – not an issue for indoor arenas, unless you water a lot.  However, “Run Off” from the Chlorides will kill vegetation and pollute our wetlands and streams.  Chlorides will cause Metal, Horseshoes, tack and other equipment to corrode. Another consequence often overlooked…. is the corrosion damage to electrical and hydraulic systems on tractors and equipment.

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!