Study: Gas-Burning Heaters Cause Ultrafine Particle Spike in Engine Plant

Research has shown that "hot" processes such as welding and high-speed machining can generate ultrafine particles (particles with a diameter less than 100 nanometers) that may be harmful to workers' health. But a new study of nanoparticle levels in an Indianapolis engine machining and assembly plant concludes that a major source of nanoparticle emissions was the exhaust from its gas-burning heating system.

The study, conducted by researchers from the University of Iowa and NIOSH, was unveiled at an Oct. 4 session of the Second International Symposium on Nanotechnology and Occupational Health, held Oct. 3-6 in Minneapolis. It looked at the ultrafine particle levels in a 1.1-million-square-foot machining and assembly facility that makes about 1,000 diesel engines a day.

Researchers involved in the study found the greatest ultrafine particle number concentrations in the block-head-rod area, and they believe that the majority of ultrafine particles generated in that area came from direct-fire, natural gas burners that heated the supply air.

Researchers observed a nearly 1,000-percent spike in number concentration of ultrafine particles in the supply air when the heating system was operating compared to when it was turned off. Researchers also noted that the number concentration of ultrafine particles in the block-head-rod area was "dramatically lower" in the spring, when the heating system was off and the outside doors were open, lead author Thomas Peters of the University of Iowa concludes in the study, titled "The Mapping of Fine and Ultrafine Particle Concentrations in an Engine Machining and Assembly Facility."

Peters, who presented the study results at the nanotechnology symposium, asserted that having a natural gas-burning, direct-fire injection heating system such as the one found in the plant "is the perfect way to generate ultrafine particles."

The study also found that processes such as high-speed machining and heat-treating operations in the cam-crank area of the plant "may produce substantial quantities of ultrafine particles" as well.

Health Risks Warrant 'Further Investigation'

The ultrafine particle number concentrations observed at the Indianapolis diesel engine plant "are likely to be representative of other machining and assembly centers," Peters suggests in the study.

Although one study asserts that 75 percent of particles between 10 and 100 nanometers will deposit within the respiratory tract, "the biological response is likely to be substantially different for particles resulting from natural-gas combustion and metalworking operations," Peters asserts in the study.

"Direct-fire, natural-gas burners are commonly used as an economical source throughout industry," Peters says. "Moreover, the enclosures used to control metalworking fluid mist [found in the Indianapolis plant] are similar to those used in other facilities. However, the risk associated with inhalation of these ultrafine particles is unclear and warrants further investigation."

The plant likely could eliminate most ultrafine particles from the block-head-rod area by finding an alternate method of heating that does not inject natural-gas exhaust into the supply air, according to the study. Peters states that a heat exchanger, while less efficient than direct-fire heating, could be used to transfer heat from gas burners "without contaminating the supply air."

In the cam-crank area, "more tightly fitting enclosures would probably be effective at controlling the ultrafine particles generated by metalworking operations."

Ultrafine Particles 'Prevalent' Throughout Facility

The study also concludes that:

  • Ultrafine particles were "prevalent" throughout the machining facility. Number concentrations of ultrafine particles inside the facility ranged from 15 to 150 times greater than outside the facility and were highly dependent on season. "Even in the assembly area where number concentrations were lowest, ultrafine particles were on average 15 times greater inside the facility than outside, regardless of the season."
  • The greatest number concentration (more than 1 million particles per cubic centimeter) occurred in the winter in the block-head-rod area, which had a low mass concentration (less than 0.10 mg per cubic meter).
  • The greatest mass concentrations were found around metalworking operations that were poorly enclosed. The larger particles that dominated particle mass in this area, according to the study, were accompanied by ultrafine particles, which "probably were generated through evaporation and subsequent condensation of metalworking fluid components."
  • Repeat analysis showed that these ultrafine particles "persist in the workplace over long time periods."

"The similarity of the number concentration maps in winter suggests that ultrafine particle concentrations may persist at elevated concentrations over time," Peters wrote. "Moreover, the relative homogeneity of number concentration within each work area suggests that ultrafine particles are easily transported by air currents from their source."

Aerosol Mapping May Be a Viable Testing Method

The goals of the study were to develop a method of mapping using "real-time instruments" to analyze fine and ultrafine particle levels in an industrial facility and to use this method to compare particle number and mass concentrations, according to the study.

Researchers used available instrumentation such as a condensation particle counter, an optical particle counter, a diffusion charging-based surface area monitor and an aerosol photometer in conjunction with a technique called aerosol mapping, which involves measuring particle concentrations throughout a facility and then relating that data through maps.

Peters added that he designed a computer program in Visual Basic to acquire the data.

The team collected its data on four different days in one week in December 2004 and one day this past March.

The engine machining and assembly facility had three primary work areas, which were open to each other without walls: the cam-crank area, the block-head-rod area and the assembly area.

  • The cam-crank area was heated by steam radiators, and its machining operations were ventilated with "relatively old, loose-fitting enclosures" that "had been retrofitted from previous use in the facility," Peters explains in the study.
  • Both the block-head-rod and assembly areas used the direct-fire, gas-burning heating system. In 2001, the plant installed "state-of-the-art exhaust enclosures" for ventilation in those two areas; however, the plant retrofitted that pre-existing heating system.

Peters concludes in the study that aerosol mapping represents a method "for the occupational hygienist to use current technology to assess fine and ultrafine particle concentrations in the workplace."

"With this technique, a general sense of the temporal and spatial variability of particles in an occupational setting may be obtained," Peters says. "It also enables the assessment of multiple metrics at one time (i.e., number, surface area or mass concentration)."

Mass Concentration Not a Good Indicator of Number Concentration

Through aerosol mapping, researchers determined that:

  • The majority of ultrafine particles in the block-head-rod area in winter may be attributed to the direct-fire heaters. Mass concentration, however, provided little or no indication of ultrafine number concentration in the block-head-rod area.
  • In the cam-crank area, processes such as high-speed machining and heat-treating operations and not the heating system may produce substantial quantities of ultrafine particles. Mass and number concentration were more closely linked in the cam-crank area, probably because metalworking operations were the source of both ultrafine and larger particles.
  • Ultrafine number concentrations were low in the assembly area despite the fact that direct-fire heaters were used there. Researchers admit this "observation is unaccounted for but may be related to differences in burner technology or air flow patterns between the two areas." Observations that ultrafine particle number and mass concentrations in the air exiting the air-handling unit were "substantially greater" when the damper was set to 100 percent outdoor air and the gas burners were off "warrant further investigation."

Overall, the study concludes that mass concentration was not a good indicator of number concentration.

"Mass concentration was low (less than 0.2 mg per cubic meter) where direct-fire heaters produced the greatest number concentration (more than 1 million particles per cubic centimeter)," Peters says. "Consequently, to the extent that health effects are related to ultrafine number exposures, current mass-based regulations may not be sufficient to protect workers in these areas."

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