This is how we have handled it at the University of Arizona:  http://risk.arizona.edu/healthandsafety/ labchemicalsafety/fumehoodexhaustsystemmaintenance.shtml

Ideally, if they are maintaining the fans and people are not using the hoods while they are working, there should be no problem.  I'd worry more about deposition of contaminants in the fans and ducts which sometimes occurs.   

But this is only true if the stacks and fans were properly designed and engineered.  Instead, what I see on the roofs of most university science buildings are rain capped exhausts or exhausts that are so low that the emissions will put workers at risk who are on the roof.  Hell, it's even worse than that--often the stuff is going right into the air handling units for the general recirculating system for the building.

If the building was built to the ACGIH Industrial Ventilation standards and to local department of environmental protection standards, those stacks would be tall enough or specially boosted to exhaust only into moving air above the building.  The height of the stacks should not be left to architects (who want no protuberances above the roof line) and engineers (who want to specify as much off the shelf equipment as possible).  Instead, the height should be determine by standards and regulations. 

If the stack heights are proper, the workers on the roof will not be exposed to the emissions.  If they are, maybe a redesign is the way to go.

Monona Rossol

In a message dated 10/30/2009 11:49:21 AM Eastern Daylight Time, stefan.w**At_Symbol_Here**UCONN.EDU writes:

In general, maintenance staff working on rooftop fume hood exhaust equipment have indicated their concerns about being on a roof, and possibly being exposed to whatever is being vented. 

In a more specific laboratory situation, involving MOCVD (metal organic chemical vapor deposition), we have a lock-out tag-out policy, due to the severity of the toxic gases involved.< br>
 

Questions:

 

Does anyone else have MOCVD labs, and to what extent are controls in place for protection of workers on roof tops?
Does anyone else employ a LOTO policy?   
Are roof tops key-accessible only?& nbsp;

Thanks for the feedback.< br>
 

-Stefan Wawzyniecki, CIH, CHMM< br>
  University of Connecticut< br>