In the USA, OSHA REQUIRES employers to "Fit Test" each worker who will wear a tight-fitting respirator.  This includes all types of tight-fitting respirators, from half-face air purifying respirators to full-face supplied air respirators.  The workers are also REQUIRED to be fit tested with EACH MAKE, MODEL STYLE and SIZE face piece they are medically qualified and trained to wear.  But it is the METHOD of fit testing that is often overlooked...

There are two types of fit testing:  Qualitative and Quantitative.  There is a HUGE difference between the two and this is where the mistake comes in.  Follow me on this...

Emergency responders, especially those trained in firefighting (1910.156), confined space rescue (1910.146(k), and hazardous materials technicians (1910.120) MUST be trained and equipped with tight fitting respirators capable of providing a protection factor of 10,000; the required protection factor for atmospheres that are Immediately Dangerous to Life and Health (IDLH).  This by default means that the respirators they MUST use will be either a:

• SCBA in the pressure demand mode (i.e. positive pressure) or 
• an air-line respirator with emergency back-up air supply (i.e. "hip air").  

It is REQUIRED that workers wearing these types of respirators be fit tested using the QUANTITATIVE method and here is the reason why:

1910.134(f)(6) QLFT may ONLY be used to fit test negative pressure air-purifying respirators that must achieve a fit factor of 100 or less. (emphasis added by me)

So with this requirement in place, the only time we can use QUALITATIVE fit testing methods is when the worker will ONLY be wearing a half-mask respirator, which by default can NOT be worn in an IDLH atmosphere.  To pass a QUANTITATIVE fit test for a full-face respirator the user MUST achieve a fit factor of 500, as stated by:

1910.134(f)(7) If the fit factor, as determined through an OSHA-accepted QNFT protocol, is equal to or greater than 100 for tight-fitting half facepieces, or equal to or greater than 500 for tight-fitting full facepieces, the QNFT has been passed with that respirator.

Lastly, be CERTAIN that EACH responder has been fit tested with EACH MAKE, MODEL STYLE and SIZE of respirator that the user has been BOTH medically quailfied and trained to use.  And if these responders will be entering into IDLH or potentially IDLH atmospheres, be sure they are wearing an SCBA in the Pressure-Demand mode (i.e. positive pressure) or an air-line respirator in the Pressure-Demand mode with an emergency air supply attached at their hip.

UPDATED on 2/25/12

I do not want to be an advertisement for any particular brand of respirators, but having used both MSA and SCOTT in my days as a Firefighter, HAZMAT tech and Rescuer I am a bit partial to these two brands so I am more informed in their products.  They both offer what I like to call "dual masks".  This is one face piece that can be used with an SCBA and as a full face air purifying respirator.  With fit-test costing around $15-30 per test, the ROI on these types of masks can be VERY ATTRACTIVE.  Businesses and FD's can save some serious $$ with an investment in these types of masks.  I have seen some businesses issue three different respirators to hundreds of employees, requiring each of these employees to be fit tested three times.  Using the low end of the price range for fit-testing this would be $45 per user which if the business had 100 users would be $4,500 each year in fit-testing costs.  This does NOT include the time spent by EACH user having to complete three fit tests.  A PROPERLY DONE fit test will take ATLEAST 15 minutes if ALL GOES WELL and there is no leakage causing an issue.  This means these employees will spend ATLEAST 45 minutes out of production mode to be fit tested each year - this too adds up to increased costs.

Tip:  I stopped using half-mask on my sites and issued one of these multi-use full face pieces.  The population who had any legitimate use for half mask was extremely small with very rare occasions.  By removing the half mask from service and moving into a full face, the workers were provided a higher protection factor and the company eliminated one additional fit test.  By issuing these new multi-use masks I went from three fit tests per employee to one fit test per employee.  This was SUBSTANTIAL costs savings each year, as well as less time the employees were in medical getting tested.  My ROI on the investment in the new mask was two years!  This also meant that ALL my ERT personnel now had their OWN face pieces and no more sharing face pieces during emergencies - ANOTHER win-win for everyone.

Here are the two products that I am most familiar with, I am sure there others, but I can not speak to them as I have not used them.

MSA Advantage® 4000 Facepiece 

Scott Quarter Turn Adapter for use with the AV-Series Face pieces (AV-2000, AV-3000, and Weld-O-Vista) to accommodate a wide variety of applications with the same face piece.