The National Institute of Standards and Technology published a a coal fly ash Materials Safety Data Sheet (MSDS) for workers exposed to the material. It says among other things:

This mixture contains fine silica dust, which can cause lung damage or cancer. It also contains metal oxides and inert dusts that may irritate the respiratory tract, skin, and eyes.

Regarding skin contact:

Remove contaminated clothing and shoes. Flush affected skin with water for at least 15 minutes, then wash thoroughly with soap and water. If skin irritation persists, get medical aid and bring the container or label. Wash contaminated clothing before reusing.

Regarding accidental release in the workplace:

Isolate the spill area. Cleanup personnel should wear personal protective equipment Sprinkle the material with water to control dust, then place in a suitable container for disposal. Provide ventilation.

Safe Handling Precautions:

Wear safety goggles and a dust mask or respirator. Avoid contact or wash after handling. Gloves or a barrier cream may be used to protect the skin.

The good news is that levels of chromium, nickel, arsenic, and other probable carcinogens in the sample tested by NIST were too low to require MSDS workplace safety documentation.

Groundwater contamination could still be a problem, though. From an EPA report on the cleanup of a fly ash Superfund Site in Virginia:

Arsenic, vanadium, nickel, selenium, and sulfate have been found in groundwater near the four fly ash pits. Surface water in Chisman Creek was shown to be contaminated with vanadium, nickel, and sulfate. Drinking contaminated groundwater posed a risk to the public. However, long-term groundwater treatment measures are in place to reduce potential risks, and residences with contaminated wells have been connected to the public water supply to ensure them a safe source of drinking water. The subsurface fly ash and pond sediment materials do not pose a public health threat in their present, covered location. Nearby estuaries which were potentially threatened by site contamination are now at significantly reduced risk.

It's sounding more and more like fly ash management and disposal needs better regulation and oversight.