These pictures will come as no surprise. But, silica dust exposures (and nuisance dust) is an ongoing issue. Bottom line: if you have dust, you need to add some controls.

Platform of rock crusher (photo courtesy BP)
silica 5

Crusher operations (photo courtesy BP)silica 6

Grinding asphalt with a Bobcat (photo courtesy AH) silica 7

NIOSH (and with the help from some other groups) released a document this last week titled, “Best Practice Engineering Control Guidelines to Control Worker Exposure to Respirable Crystalline Silica during Asphalt Pavement Milling”cold milling machine

The issue: These machines are used to remove asphalt roads. They have a drum with teeth on them that essentially chew up the road and asphalt. A lot of respirable silica is generated (based upon the amount of silica in the rocks). The drums get really hot so water is used to cool it.  However, it does not control the respirable silica dust.  I’ve written (or, maybe complained) about the issue here, in 2010. And, I was informed, some good people were working on it.

The solution: The quick summary is: add more water and ventilation. Not rocket science, right? However, after reading this document, it might be. There’s a lot of information and specifics on what worked, and what didn’t. It was almost too much detail, but I suppose if you have a $200k+ machine, it is worth the time to figure it out. Below are some details:

  • Case studies – adding water and increasing the pressure flow decreased airborne dust
  • Tracer gas studies for ventilation effectiveness
  • Checklists and flow rate controls
  • Diagrams for where to direct water

Another benefit was the documentation of other’s work. There are numerous references  (5 pages!) to scientific articles. I did not notice any cost to implement the recommended changes, and I am curious to know what adding the ventilation system might run. Overall the document is good.

Finally, if you hold-on and continue reading to Appendix C, let me know what that all-means.  🙂



It is officially summer and construction road crews & roofing is in full swing. Some projects require the use and application of coal tar pitch. Not only is it stinky, it is is hazardous.

Here’s some info:

  • Uses
    • Roofing
    • Asphalt seal coating
    • Pharmaceutical treatment for psoriasis (scalp/skin condition)
    • Graphite industry (in the production of graphite)
  • General
    • Coal tar pitch is actually a make-up of a bunch of different substances (maybe even 10,000 of them)
    • Contains lots of polycyclic aromatic hydrocarbons (PAHs) and other chemicals including: benzene, pyrene, benzo(a)pyrene, phenanthrene, anthracene
  • Exposure
    • can be exposed by inhalation, ingestion (is this likely?), or exposure to skin, eyes
    • considered a carcinogen if the product contains more than 5% of coal tar
    • cancers include: skin, scrotal, lungs, bladder, kidney & digestive
    • increases your sensitivity to sunlight (easier to sunburn)
  • Safety
    • Pick a sealant/coating that does not contain coal tar. A list of some can be found here.
    • Avoid inhalation & skin/eye contact
    • Train your employees. A sample safety SDS (MSDS) can be found here.
    • Wear the correct PPE.
    • Air sample to determine exposures. OSHA has a method (58).
  • Resources


For people who work with asphalt everyday, this may be a obvious statement: Silica is in asphalt.

There are two areas where this might be of concern:


  1. During the mixing of the asphalt there is usually some ratio of rocks that are added to the product. The size of these rocks is where safety personnel should have concerns. If they are adding the smallest fraction of rock to the mix, you may have airborne exposures to silica. This size of rock is called P200 (or, using a 200 sieve screen to get anything less than 200 parts per inch). Usually this is seen during the transfer of rock from the loader or from the belt conveyor.
  2. The other method of exposure to silica in asphalt is if you cut it. Rocks may contain silica. When asphalt is cut, you incidentally cut through various rocks (contained within the asphalt). Silica (and dust) is released during the cutting. Please use water when you are cutting asphalt. Direct the water at the blade.