Industrial hygiene in Flagstaff: what 7,000 feet does to a survey

Flagstaff sits near 7,000 feet, where air pressure runs around 78% of what it is at sea level. That is more than scenery, because it changes what an air sampling pump actually measures.

Our field is industrial hygiene, the discipline behind workplace air monitoring and noise surveys.

What altitude does to an air survey

It does four things, listed here in order of how often they go wrong.

1. The pump has to be calibrated on site, against a primary standard. One common flow meter, the rotameter, gives a reading that depends on air density. Suppose a rotameter was calibrated near sea level. If you set a pump against it and then run that pump at 7,000 feet, the flow the meter showed is not the flow you actually get.

Applying a correction factor afterward is not the fix. The fix is to calibrate the pump where the sampling happens, at the temperature and pressure of the site itself. That takes a primary standard, the top grade of flow meter, such as a frictionless piston meter or an electronic bubble meter. The meter also has to be rated for the altitude, because several common units are rated only for a stated pressure range.

2. The respirable cyclone performs differently. A respirable cyclone separates out the fine dust that can reach deep into the lungs. Its separation point, called the 50% cut point, depends on the density and viscosity of the air as well as the flow rate.

Take a 10-mm nylon or aluminum cyclone and run it at 7,000 feet at the sea-level flow rate printed on it. You do not get the same cut point of 4 micrometers (µm) that you get in Phoenix. When a respirable dust result matters, as with wood dust, silica or other fine dust, the cyclone's flow has to be set for the conditions, not for the label.

3. The correction goes in the flow, not in the unit conversion. This is the subtle one. Results can be reported in milligrams per cubic meter of air (mg/m³) or in parts per million (ppm), and converting one to the other uses a standard gas volume. That volume is defined at 25 °C and 760 mmHg of pressure, and OSHA writes its limits on that same basis.

If you adjust the ppm calculation for local conditions, the result can no longer be compared with OSHA's permissible exposure limit (PEL). The altitude correction belongs in the volume of air sampled, not in the unit conversion.

4. Gas meters read low. Many gas meters use an electrochemical sensor cell, and the cells for carbon monoxide, hydrogen sulfide and oxygen respond to pressure. A cell calibrated at low elevation therefore reads roughly in step with the air pressure around it. At 78% of sea-level pressure, a meter that has not been corrected reads meaningfully low.

That is why an oxygen meter has to be set in fresh air on site, at elevation, every time, a step known as a fresh-air zero. For toxic gas alarms in a confined space, this is about saving lives, not paperwork.

None of this makes sampling at elevation difficult, but it does have to be planned before the drive up I-17. A Flagstaff job is not a Phoenix job with a longer drive, and we do not treat it as one.

→ Air monitoring surveys · Respirable dust monitoring

The industry up here

Medical device and precision manufacturing is Flagstaff's largest group of industrial jobs. The work includes machining, polymer processing, coating and cleanroom assembly, and solvent and adhesive exposures come with it.

Forest products. Northern Arizona's ponderosa pine supports sawmills, millwork, and forest thinning and restoration work. That brings up a fact most shops have been told incorrectly: OSHA has no PEL written for wood dust.

OSHA set wood dust limits in 1989, but a federal appeals court struck them down (AFL-CIO v. OSHA, 965 F.2d 962, 11th Cir. 1992). Wood dust therefore falls under OSHA's catch-all dust limits, a group OSHA calls Particulates Not Otherwise Regulated. Those limits are 15 mg/m³ for total dust and 5 mg/m³ for respirable dust.

Two other organizations set lower numbers:

• NIOSH sets its recommended exposure limit (REL) at 1 mg/m³ and classes wood dust as a potential occupational carcinogen.
• ACGIH, the professional body that publishes the threshold limit values (TLVs), is at 1 mg/m³ for most kinds of wood, measured as inhalable dust.

→ Woodworking and millwork shops

The air agency is the state, not a county

This is worth stating because the Phoenix and Tucson answers do not apply here. Only three Arizona counties run their own air programs: Maricopa, Pima and Pinal. Coconino County is not one of them, so air permits for Flagstaff sites come from ADEQ, the Arizona Department of Environmental Quality.

We do not handle air permits. If you keep a compliance obligations register (the list of laws and rules you must follow), though, do not copy it from a Phoenix plant, because it will point to a county office that has no role here.

→ How to build a compliance obligations register

Getting there

We work from one Arizona office and have no Flagstaff branch. Flagstaff is about 2.5 hours up I-17, and we plan around that. We calibrate equipment at your site, not before we leave, and we schedule a survey to finish in consecutive days, not in scattered visits.

We work in four fields here:

• Tier II chemical inventory reporting (due March 1)
• Form R / TRI reporting (due July 1)
• Air monitoring surveys and noise surveys
• ISO consulting for environmental management (ISO 14001) and for occupational health and safety (ISO 45001)

→ Where we work