The airline and aerospace industry is one of the few that still uses chromate-containing primers, such as Mil-P-23377 "Primer Coatings: Epoxy, High-Solds." This primer contains strontium chromate, but some facilities still use coatings that contain zinc chromate for painting ground support equipment and other applications.

Since the early 1990s, I have visited numerous aerospace facilities, including Air Force bases, OEMs and contractor facilities, and with few exceptions I have found the paint spray booths and paint hangars to be poorly maintained with regard to overspray. In my opinion, the new Chromium VI rule will now force painting facilities to thoroughly review the procedures they follow to minimize overspray from chromate-containing coatings. An unintended, yet excellent consequence will be that paint shops will be cleaner regardless of whether or not the coatings contain Chromium VI.

The Chromium Rule
On Oct. 1, 2004, the Federal Register gave an in-depth review of the Proposed Rule: Occupational Exposure to Hexavalent Chromium; 29 CFR Parts 1910, 1915, 1917, 1918, and 1926.

This author has selected only two paragraphs to illustrate the problem of hexavalent chromium (chromium VI) as it pertains to painting operations:

"Regular exposures at higher levels, including the current PEL of 52 µg/m³ Cr(VI), are expected to cause substantially more deaths per 1,000 workers from lung cancer than result from occupational injuries in most private industry. At the proposed PEL of 1 µg/m³ Cr(VI), the agency's estimate of excess lung cancer mortality falls much closer to the private industry average fatal injury rate, given the same employment time, but still exceeds the rates found in lower-risk industries such as finance and health service.

" … , in painting operations the exposure is to chromate pigments with moderate and low solubility such as zinc chromate, strontium chromate and lead chromate; in sanding and polishing operations the same chromate pigments exist as dust; while platers and tank tenders are exposed to chromium trioxide, which is highly soluble."

From these two paragraphs alone we can see that painters and those who work immediately outside paint spray booths are at risk of suffering cancer if they are exposed to even small concentrations of chromium VI. Although the proposal has been on the books since 2004, the industry has long awaited publication of the final rule. The following statement appears on the OSHA Web site¹: "On Jan. 17, 2006, the U.S. Court of Appeals for the Third Circuit granted a six-week extension to OSHA to publish a final rule for occupational exposure to hexavalent chromium. The new deadline for publication is Feb. 28, 2006. The agency requested the extension due to unanticipated delays as a result of Hurricane Katrina and the subsequent activation of OSHA's Worker Safety and Health Support Annex to the government’s National Response Plan."

At the time of this writing, the final rule was only a few days from being published. But it is evident that the old chromium VI standard of 52 µg/m³ Cr(VI) will be significantly reduced.

This article will not go into the various arguments that have been raised by the industry, concerning the low threshold limit, but rather it will explore methods for dramatically reducing the amount of chromium VI that deposits on the spray booth walls, floor, and ceiling. At least within the confines of a spray booth, the painters are already expected to wear personal protective equipment (PPE), such as particulate and vapor respirators, gloves, boots, and coveralls. The problem that thousands of painting operations will face after the rule takes effect is how to keep chromium VI from entering the general workplace where non-painting operators, technicians, assembly line workers, janitors, and office staff might be exposed to concentrations > 1 µg/m³ Cr(VI).

In this article, I will illustrate how a typical paint spray booth can cause this problem.

Paint Spray Booths
Over the past five years, I have performed numerous spray booth audits to get a sense of airflow and turbulence. Heretofore, poor airflow and turbulence were considered to affect transfer efficiency of paint application, deposition of dirt and dust on freshly painted surfaces, excessive loading of paint filters (or water wash systems), and the settling of overspray on hoses, fittings, regulators, and pressure gauges. While this has always been recognized to be a terrible nuisance, I have never encountered painters, supervisors, and environmental staff who have complained about the potential health problems caused by the excessive overspray. The new chromium VI rule will change all that.

Figure 1: Side draft spray booth. (Click on image to enlarge).

Not all paint facilities use chromate-containing primers and topcoats, but I urge all readers of this article to take note of the findings and recommendations, because implementation will lead to better, safer, and perhaps even leaner operations.

Air Velocity and Turbulence
Figure 1 shows a typical three-sided (open-backed) dry filter, side draft spray booth. To perform an air velocity profile, several locations within the spray booth were selected where velocity readings were to be measured. For the spray booth in this article, I identified six locations at which the measurements were be taken. Points 1 and 4 were three feet from the left wall. Points 2 and 5 were along the centerline, and points 3 and 6 were three feet from the right wall.

The three points closest to the filters, namely points 4, 5, and 6, were three feet from the filters, while the remaining three points were three feet from the back of the booth. All measurements were taken at a height of four feet above the floor, because predominantly small parts were painted on table tops at this level.

Figure 2: Open-backed side draft spray booth with table fans turned on.

Figure 3: Open-backed side draft spray booth with table fans turned on.

The spray booth in this article was located inside an environmentally uncontrolled assembly area of an electronics shop where table fans were placed next to the workers predominantly to keep them cool. There were so many table fans near the spray booth that they had an effect on the airflow inside the booth.

The air velocity survey for which the results will now be presented took place toward the end of the work shift. Figures 2 and 3 illustrate how the air velocity changed at each of the six positions over a period of 100 seconds. The wildly fluctuating, zig-zag graphs show that with all of the fans operating, the air velocity, especially at points 1, 2, and 3 was unbelievably turbulent. Points 1 and 4, close to the left wall, were by far the most turbulent, flowed by the two centerline points. Points 3 and 6, near the right wall, were considerably less turbulent.

The same test was repeated at the end of the shift when the assembly workers departed for the day and I could turn the fans off. Now the results showed some improvement, although by no means was the turbulence eliminated.

In a letter dated Oct. 22, 2001, OSHA responded to a question regarding air velocity in a spray booth as follows²:

Figure 4: Open-Backed side draft spray booth with table fans turned off.

Figure 5: Open-Backed side draft spray booth with table fans turned off.

Figure 6: Air velocity at face of filters.

"Table G-10 of 29 CFR 1910.94(c)(6)(i) specifies the required airflow velocities ranging from 50 to 250 linear foot per minute (fpm) into the openings of a spray booth for various operations and designs, except where a spray booth has an adequate air replacement system. The standard 1910.107(b)(5)(i) is specifically for dry type overspray collectors, which are required to maintain the average air velocity of 100 linear fpm over the open face of the booth or booth cross section during all spraying operations except electrostatic spraying."

Figures 4 and 5 clearly show that for the spray booth of this report, the air velocity was well below 100 fpm in most locations, even at points 4, 5, and 6, which were closest to the exhaust filters.

Of all the spray booths in which I have performed such measurements, fewer than 5% have passed the velocity criterion; the spray booth of this report was no exception.

Measuring Velocity at the Filters
To better understand what was happening, I measured the air velocity within 1/2 in. of each filter. Figure 6 provide the results. There were 17 filters along the top, each 20 x 20 in., and similarly 17 filters in the middle and bottom rows, respectively. The findings were as follows:

• For all rows, air velocity was poorest along the left wall and slowly increased towards the right wall;
• air velocity through the filters along the bottom row was poorer than for the middle and top rows, respectively;
• the velocity trends were almost identical regardless of whether the table fans were turned on or off;
• and the average velocity was 178.1 fpm when the fan was turned on, and 169.4 fpm with the fan turned off. This difference is considered to be insignificant. Had the fan pushed more air into the filters, the corresponding velocity would have been much higher.

None of these results were surprising. It appears that the exhaust fan on the left was not pulling as intended, and should be inspected for a malfunction. The fact that the bottom row of filters was more plugged than the filters above is totally understandable. The painters paint small parts on tables and the spray gun is approximately four feet above the floor, therefore, it is unlikely that much overspray will leave through the top row of filters.

Conclusion and Recommendations
The table fans were effective in providing ventilation and cool air to the assembly workers, but the unintended consequence was that the overspray containing chromium VI escaped from the back of the booth and was not captured by the filters. In fact, a short inspection of the floor immediately outside the booth and up to several feet outside the booth showed evidence of the primer.

Paint operators are advised to perform air velocity and air turbulence surveys to better understand what is happening to the toxic overspray. These booths must now operate under slightly negative pressure to prevent overspray from escaping into the general work area.

Painting operations that use chromium VI primers and topcoats, and apply them in open-backed booths, should seriously consider enclosing them. Two options present themselves; install filtered doors or add an air supply system to the booth. Of the two, the filtered doors are less expensive. Moreover, the resistance offered by the filters in the doors almost guarantees that the booth will operate under negative pressure.

Air supply systems have the opportunity to provide more air flow through the booth, but if the air supply fan and air exhaust fans are not balanced, the booth might tend to operate under positive pressure. Even if, initially, the booth is under negative pressure, it might turn positive unless a control, such as variable frequency drive (VFD), is installed.

Turbulence will now pose more of a regulatory problem than before, because when you inspect such booths you will invariably find excessive overspray dust on the floor and walls. Facility owners who allow their painters to work in this environment without wearing appropriate PPE at all times run the expensive risk of violating the chromium standard. It will be a daunting task to safeguard the health and safety of painters not only while they are spray painting, but also while they are masking or preparing parts for painting.

References

1. http://www.osha.gov/SLTC/hexavalentchromium/index.html
2. http://www.osha.gov/pls/oshaweb/owadisp.show_document?
   p_table=INTERPRETATIONS&p_id=24001

Ron Joseph is an independent coating consultant in San Jose, Calif.
You can e-mail questions to drrojo@aol.com.