Characterizing Exposures to Airborne Metals

and Nanoparticle Emissions in a Refinery

ARTHUR MILLER*,

*Author to whom correspondence should be addressed. 

Tel: þ1-509-354-8028; fax: 1-509-354-8099; 

e-mail: ALMiller@cdc.gov 

þ

PAMELA L. DRAKE, PATRICK HINTZ and

MATT HABJAN

National Institute for Occupational Safety and Health, Spokane Research Laboratory,

315 E. Montgomery Ave, Spokane, WA 99207, USA

An air quality survey was conducted at a precious metals refinery in order to evaluate worker

exposures to airborne metals and to provide detailed characterization of the aerosols. Two

areas within the refinery were characterized: a furnace room and an electro-refining area.

In line with standard survey practices, both personal and area air filter samples were collected

on 37-mm filters and analyzed for metals by inductively coupled plasma–atomic emission spectroscopy. In addition to the standard sampling, measurements were conducted using other

tools, designed to provide enhanced characterization of the workplace aerosols. The number

concentration and number-weighted particle size distribution of airborne particles were measured with a fast mobility particle sizer (FMPS). Custom-designed software was used to

correlate particle concentration data with spatial location data to generate contour maps of

particle number concentrations in the work areas. Short-term samples were collected in areas

of localized high concentrations and analyzed using transmission electron microscopy (TEM)

and energy dispersive spectroscopy (EDS) to determine particle morphology and elemental

chemistry. Analysis of filter samples indicated that all of the workers were exposed to levels

of silver above the Occupational Safety and Health Administration permissible exposure limit

of 0.01 mg m23 even though the localized ventilation was functioning. Measurements with the

FMPS indicated that particle number concentrations near the furnace increased up to 1000­

fold above the baseline during the pouring of molten metal. Spatial mapping revealed localized

elevated particle concentrations near the furnaces and plumes of particles rising into the stairwells and traveling to the upper work areas. Results of TEM/EDS analyses confirmed the high

number of nanoparticles measured by the FMPS and indicated the aerosols were rich in metals

including silver, lead, antimony, selenium, and zinc. Results of the survey were used to deduce

appropriate strategies for mitigation of worker exposure to airborne metals.

INTRODUCTION

Some of the intrinsic properties of metallic elements, 

such as electrical conductivity, hardness, or catalysis, 

have made metals an integral part of modern industrialized society. Other properties render the metals 

toxic, with potential to impact the health of workers 

that mine them (Brune et al., 1980; Thomassen 

et al., 1999) or prepare them for use by the rest of society (Linnainmaa et al., 1996; Chadwick et al., 1997; 

Zimmer and Biswas, 2000; Drake and Hazelwood, 

2005; Harris et al., 2005). In an effort to minimize 

the health impacts of metals on workers, particularly 

airborne metals, occupational exposure limits have 

been developed to minimize or mitigate the hazards 

posed by workplace exposures (OSHA, 1989; NIOSH, 

2005; ACGIH, 2008). In order to appropriately measure workers’ exposure to airborne metals, standard 

methods of analysis have also been developed. The 
International Organization for Standardization (ISO) 

method 15202 (ISO, 2000; ISO, 2001; ISO, 2004) 

was designed for analyzing air filter samples for metals and metalloids, including soluble compounds, 

whereas the National Institute for Occupational 

Safety and Health (NIOSH) method 7303 (NIOSH, 

2003) was designed for measuring total metals on 

filter samples. 

Analytical methods usually include protocols for 

the collection of representative samples to determine 

worker exposures. Typically, the standard methods 

describe collection of filter samples for specified periods of time in the breathing zone of individual 

workers while they are performing their normal duties. Area filter samples in selected work areas are 

often collected as well, and both types of samples 

are analyzed for the contaminant of interest. Researchers continue to investigate the validity and quality of the samples collected that evaluate a worker’s 

breathing zone (Clinkenbeard et al., 2002; Ashley 

et al., 2003). Occasionally, additional data are gathered using alternative techniques such as the collection of real-time samples to deduce task-specific 

exposures (Martinelli et al., 1983), continuous automated sampling to provide round-the-clock data acquisition (Samanta et al., 2001), or the generation of 

spatial maps from the data (Evans et al., 2008). 

There is considerable toxicity information available regarding airborne metals; however, the effect 

of the particle size on biological responses is only recently becoming better understood (Cheng, 2004; 

Sivulka et al., 2007; Nurkiewicz et al., 2008). The 

need to measure the size distribution of the particles 

during workplace surveys is becoming increasingly 

important as awareness increases and more information regarding the hazards of ultrafine particles becomes available, since they have the potential for 

enhanced bioavailability and thus detrimental health 

effects (Oberdoerster, 2001; Ibald-Mulli et al., 

2002). In response to this concern, NIOSH has begun 

to focus some of its research on the development of 

new techniques for workplace surveys, particularly 

regarding characterization of nanoaerosols. 

In the current work, the authors set out to demonstrate the use of additional tools to enhance the results of a standard workplace survey, with a focus 

on developing a robust yet streamlined and portable 

system for evaluating airborne hazards in the workplace. The approach employs three distinct tools. 

The first of these tools entails making workplace 

measurements of particle number concentration 

using a TSI Model 3091 Fast Mobility Particle Sizer 

(FMPS), along with the use of custom-designed 

software for logging the data and generating twodimensional tomographic maps of the information. 

The second tool involves using a data-logging photometer to gather time-resolved information about 

airborne particle mass concentration. The third tool 

entails ‘informed sampling’, i.e. the collection of 

particulate samples at key locations in the workplace 

using a handheld electrostatic precipitator (ESP) and 

subsequent detailed characterization of the aerosols 

via analysis of collected samples using transmission 

electron microscopy (TEM) and energy dispersive 

spectroscopy (EDS). By applying these tools along 

with the standard filter sampling approach, we were 

able to generate additional valuable information regarding the number, size, morphology, and fate of ultrafine workplace aerosols. 

To evaluate this new approach, a workplace survey 

was conducted in which these tools were used side-byside with the standard (filter sampling) method in 

order to evaluate the potential benefits of the approach. 

The survey entailed evaluating worker exposure to 

airborne metals resulting from operation of furnaces 

and an electro-refining process during purification of 

silver at a precious metals refinery. 

METHODS

Process description

The precious metal processing facility uses a threestage process to refine silver. A high-volume furnace 

is used to melt the incoming feedstock, which is metallic silver with some impurities. This furnace is located on the first floor of the building and employs 

a local ventilation system to control worker exposures 

by capturing furnace fumes using an airflow drawn 

through a cowl installed around the furnace. Up to 

1200 kg of silver feedstock per batch are melted 

slowly over a period of a few hours. Subsequently, 

over a period of ~50 min, the molten metal is poured 

into molds which have been preheated with a propane 

torch. The molds are specially shaped to form plates 

of silver, which become the anodes in the electrorefining process used to purify the silver. After cooling, these anode plates are transferred to the second 

level of the building, where they are placed opposite 

from stainless steel cathodes in an ion exchange cell, 

consisting of a nitric acid bath. A voltage potential is 

applied across the electrodes that oxidizes the impure 

silver anodes and grows silver crystals on the stainless 

steel cathodes in each of the individual ion exchange 

cells. After deposition is complete, the silver crystals 

are removed from the cathodes and transported back 

to the high-volume furnace to be made into anodes 

for further refining or to a low-volume furnace, also 

located on the main floor, to be melted and molded 
into ingots, which is the final product from this facility. The low-volume furnace is located in a large local 

ventilation plenum, which draws the fumes away 

from the workers. 

Filter sampling

Filter samples were collected to determine the 

workplace levels of metals, including both total silver 

and soluble silver compounds. In the USA, the Occupational Safety and Health Administration (OSHA) 

established a permissible exposure limit (PEL) of 

0.01 mg m-3 for all forms of airborne silver. The 

NIOSH also set their recommended exposure limit 

(REL) for silver at 0.01 mg m-3 (OSHA, 1989; 

NIOSH, 2005). However, the American Conference 

of Governmental Industrial Hygienists (ACGIH) established a threshold limit value (TLV) of 0.1 mg m-3 

for metallic silver and 0.01 mg m-3 for soluble silver 

compounds (ACGIH, 2008) (Table 1). Although no 

OSHA PEL exists for soluble silver compounds, it 

is more toxic than metallic silver (Drake and 

Hazelwood, 2005) and the ACGIH TLV for metallic 

silver, while not a formal standard, is 10 times higher 

than the PEL for total silver. In order to differentiate 

between the two forms of silver, two analytical 

methods were used in this study. The NIOSH method 

7303 (NIOSH, 2003) was used for deriving total 

metals, including silver, and the ISO method 15202 

(ISO, 2000; ISO, 2001; ISO, 2004) was used to analyze some of the area filter samples for soluble silver 

compounds. 

Personal samples were collected over a full work 

shift on six employees, two working on the electrorefining floor and four working on the furnace floor 

(Table 1). 

Table 1. Occupational exposure limits for the metals detected by NIOSH method 7303, along with airborne metal concentrations 

(mg/m3

) derived from personal filter samples. Concentrations in bold are above the OSHA PEL. Letters A–N indicate the 

location of the area samples in Figs 1–5. 

Antimony Copper Iron Lead Silver Soluble silver Zinc 

OSHA PEL 0.5 0.1 10 0.05 0.01 5 

NIOSH REL 0.5 0.1 5 0.05 0.01 5 

ACGIH TLV 0.5 0.2 5 0.05 0.1 0.01 2 

Personal sample results 

Electro-refining floor a 0.071 0.023 0.003 0.049 0.010 

Electro-refining floor a 0.095 0.045 0.004 0.094 0.001 

High-volume furnace operator 0.003 0.013 0.017 0.006 0.029 0.001 

Furnace floor, forklift operator 0.003 0.006 0.006 0.004 0.013 0.001 

Chemistry lab; furnace floor a 0.008 0.009 0.002 0.038 0.001 

Low- and high-volume furnace operator 0.002 0.007 0.007 0.005 0.018 0.001 

Area sample results 

Furnace floor 

A—near high-volume furnace 0.003 0.006 0.002 0.005 0.017 0.001 

B—between furnaces 0.002 0.006 0.005 0.005 0.013 0.001 

C—across from high-volume furnace 0.003 0.008 0.003 0.009 0.015 0.001 

D—across from low-volume furnace 0.003 0.007 0.003 0.007 0.017 0.001 

E—15 m from furnaces 0.003 0.006 0.002 0.005 0.013 0.001 

F—near low-volume furnace 0.003 0.007 0.003 0.005 0.017 0.001 

G—near stairwell 0.005 0.012 0.002 0.015 0.022 0.001 

H—near low-volume furnace a 0.006 0.005 0.004 0.039 0.002 

Electro-refining floor 

I—above ion exchange cells a 0.008 0.003 a 0.006 0.002 0.001 

J—above ion exchange cells a 0.011 0.002 a 0.006 0.001 0.002 

K—above ion exchange cells a 0.012 0.003 a 0.007 0.002 0.002 

L—above ion exchange cells a 0.009 0.002 a 0.006 0.001 0.001 

M—15 m from ion exchange cells a 0.005 0.002 a 0.004 0.001 

N—cell maintenance, cleaning a 0.031 0.006 0.004 0.012 0.005 

a

Below the limit of quantification. 

Each sample was collected on a 37-mm 

mixed cellulose ester filter, housed in a closed-face 
plastic sampling cassette. Using plastic tubing, the 

cassette was connected to a medium flow-rate pump, 

which was attached to the worker’s belt and calibrated 

at 2.0 l min -1

. The cassettes were clipped to the workers’ collars so that air from their breathing zone could 

be collected. All personal samples were analyzed using the NIOSH method 7303 (NIOSH, 2003). This 

method entails a strong acid digestion of the filter 

samples and analysis by inductively coupled 

plasma–atomic emission spectroscopy (ICP–AES). 

Concurrently with personal sampling, 8 area samples were also obtained on the furnace floor and 10 

on the electro-refining floor to evaluate exposure 

zones and the range of airborne metal concentrations 

throughout the refinery (Table 1 and Fig. 1–5). The 

area samples on the furnace floor and six of the samples from the electro-refining floor were analyzed using NIOSH method 7303 (NIOSH, 2003). The 

remaining four samples from the electro-refining floor 

were collected on 37-mm polytetrafluoroethylene filters housed in an opaque closed-face plastic sampling 

cassette and analyzed using ISO Method 15202 for 

soluble silver (ISO, 2000; ISO, 2001; ISO, 2004). In 

this method, deionized water is used to extract the soluble metal compounds followed by ICP–AES analysis. The levels of soluble silver were of particular 

interest in this study due to the possibility of aerosolization of the liquid in the electro-refining process. 

Aerosol characterization

Airborne particle concentrations (particles per cubic centimeter air) on the furnace floor and the electro-refining floor were measured throughout the day 

using a FMPS mounted on a cart that was moved to 

a new location approximately every minute. The 

FMPS took a continuous sample at a vertical height 

equivalent to a worker’s breathing zone and measured the size distribution of particles between 5.6 

and 560 nm at a frequency of 1 Hz. 

Data were collected by the FMPS for 1 min at 

each location and synchronized with location data recorded into a laptop-based mapping system. The mapping system comprised custom software designed to 

allow the user to import a plan view map of the workplace, click a location on the map, and link data to that 

location. The combined data were used to generate 

contour maps of particle number concentration, based 

on a distance-weighted interpolation scheme between 

data points (Fig. 1–5). Each map represents data gathered at 12–20 selected locations, spaced 3 m apart 

throughout each floor of the refinery. The number 

and spacing of sampling locations were chosen to allow measurement at all locations on each floor over 

a total time period of 15–30 min, so the maps represent ‘snapshots’ of particle concentration in the workplace during that time period. This mapping technique 

was used to investigate the fate of airborne particles 

within the plant and to help identify areas of higher 

concentrations for more focused short-term sampling 

and particle characterization. 

Short-term particle samples were collected over 

5–50 s periods at selected locations, identified during 

the mapping process, using a handheld ESP being 

developed by the authors. Each sample from the 

ESP consisted of particulate deposition onto a 400 

mesh copper TEM grid coated with SiO film, which 

was analyzed for particle size and morphology 

(shape) using a TEM. The samples were also analyzed using EDS to provide the elemental content 

for a number of selected particles. The analyses were 

done using a Philips CM12 microscope equipped 

with a Power MX EDS system (EDAX, Inc., Mahwah, NJ, USA). 

Real-time data regarding the mass concentration 

of airborne particles were also collected throughout 

the day, using two light-scattering photometers. 

The photometer chosen for this survey was the Thermo Electron Personal DataRAM 1200 (PDR). The 

PDR is a small lightweight instrument designed to 

be attached to a workers belt and operated as a passive sampler. It can also be used as an active sampler 

for taking area samples as was done for this survey. 

For active sampling, an auxiliary pump was used to 

draw air over the sensor, and a cyclone (BGI Model 

GK 2.05) was attached to remove larger particles, 

i.e. those .5.5 lm. The PDR gives the best sensitivity in the respirable range, is auto ranging between 

0.001–400 mg m -3

, can sample as fast as 1 Hz, 

and has built-in data-logging capability. 

Two PDRs were used to collect area-based data, 

one located on the upper level between two of the 

ion exchange cells (next to area filter sample J in 

Figs 4 and 5) and one on the lower level near the 

high-volume furnace (next to area filter sample A 

in Figs 1–3). The PDRs are factory calibrated using 

SAE Fine (ISO Fine) dust, and thus, the measured 

data do not accurately reflect the mass concentration 

of the metallic particles. However, data from the two 

PDR photometers do provide a valuable indicator of 

trends in approximate mass concentration. 

RESULTS

Filter samples

Of the many elements analyzed on the personal 

filter samples using the NIOSH Method 7303 
(NIOSH, 2003), only a small number had concentrations above the quantification limit for the method 

(Table 1). The only metal with concentrations in 

excess of the NIOSH REL and the OSHA PEL 

was silver. Similarly, analysis of the area samples 

showed that the range in mass concentration of total 

silver was 0.013–0.039 mg m -3 on the furnace floor 

and 0.004–0.012 mg m -3 on the electro-refining floor, 

while levels of all other metals were significantly less 

than the PELs (Table 1). All area samples analyzed for 

soluble silver compounds were ,0.002 mg m -3

. 

Aerosol mapping

Airborne particle number concentration data were 

collected beginning at 8:00 AM while the highvolume furnace was warming up. The mapped data 

(Fig. 1) show that particle concentrations were slightly 

higher around the furnace, but overall, the concentrations were relatively low. 

Fig. 1. Map of particle number concentration generated at 

start of shift (8:00 AM), which constitutes baseline data for the 

furnace floor. Letters A–H are the locations for the area filter 

samples. That initial survey provided 

a baseline level of particle number concentration for 

the furnace floor, in the range of 5 103 – 

5 x 104 particles cm -3

. 

x

The particle number concentration on the main 

floor rose as the high-volume furnace heated up 

and the feedstock began melting. During the pouring 

of the molten feedstock into the anode molds, the 

particle concentrations reached the highest level 

and the peak of the particle size distribution decreased from a mean diameter of 100 nm for baseline measurements to 10 nm near the furnace. 

The map generated at the beginning of the first pour 

(Fig. 2) shows concentrations .106 particles cm -3 

around the furnace and anode mold areas. 

Fig. 2. Map of particle number concentration generated 

during the first pour of molten silver into an anode mold. 

Letters A–H are the locations for the area filter samples. 

Particle number concentration data were also collected during the pouring of refined silver ingots. 

The map generated 2:00 PM during the pouring 

of ingots is shown in Fig. 3 and indicates that the 

areas of high particle concentration were again limited, in this case to the region immediately adjacent 

to the low-volume furnace and ingot mold trays. 

Fig. 3. Map of particle number concentration during pouring 

of refined silver ingots. Letters A–H are the locations for the 

area filter samples. 

A map of the particle number concentration on the 

electro-refining floor, generated from data collected 

at 9:00 AM (Fig. 4), shows particulate levels similar to the baseline levels on the furnace floor. Data 

collected at 12:00 PM (Fig. 5) show slightly higher 

concentrations overall and of particular note are the 

stairwells (on the far right and lower left in Fig. 5), 

which appear to serve as pathways for particles from 

the furnace floor to migrate upward into the electrorefining floor. 

Aerosol characterization

The particle number and size distribution measured at 11:30 AM by the FMPS during the pouring of molten feedstock into the anode molds show 

high numbers of nanoparticles (Fig. 6). ESP samples 

taken near the high-volume furnace during that 

time period, analyzed by TEM, show the typical 
morphology of the particles (Fig. 7) and confirm the 

high number of nanoparticles measured by the 

FMPS. Analysis of these particles by EDS indicated 

that they are mainly metallic, including elements 

such as silver, lead, selenium, antimony, and zinc, 

as well as carbon and chlorine and that the relative 

amount of each element varied considerably from 

particle to particle. An example of an EDS spectrum 

for a typical particle collected near the high-volume 

furnace is shown in Fig. 8. 

Fig. 4. Map of baseline particle number concentration on the electro-refining floor at 9:00 AM. Letters I–N are the locations for the 

area filters samples. 

Fig. 5. Map of particle number concentration on the electro-refining floor at 12:00 PM. Letters I–N are the locations for the area 

filters samples. 

Fig. 6. Typical particle size distributions before and during the 

pouring of molten silver into anode molds and refined silver 

ingots. 

Similarly, FMPS data and ESP samples were collected during the pouring of refined silver ingots. 

The FMPS data (Fig. 6) show somewhat lower number concentrations than measured during the anode 

mold pour and again show that the particles are in 

the nanometer size range. TEM images from particles collected during pouring of refined silver show 

similar morphology and uniformity as the particles 

collected during the anode pouring process, as well 

as a high proportion of nanoparticles. The EDS data 

from individual particles, such as the spectrum in 

Fig. 9, show less variability than for particles generated during the anode pouring and also indicate higher levels of silver and less impurities. 

Data from the two PDR photometers provide 

a valuable indicator of trends in approximate mass 

concentration in the workplace throughout the day 

(Fig. 10). The data in Table 2 show which events 

throughout the day resulted in increased levels of 

particulate matter in the air. The furnace floor monitor was near the high-volume furnace and thus 

shows very high values during the mold-warming 

task 10:50 AM (due to use of propane torches) 

and during the anode pour 11:20 AM. 
Fig. 7. TEM image of particles collected during the pouring of 

molten silver into an anode mold. 

Fig. 8. EDS spectrum from a typical single particle collected 

near the high-volume furnace, showing contaminants 

including lead, zinc, and antimony. Note: This EDS spectrum 

is ‘uncorrected’ and the peaks from Cu, Si, and O must be 

assumed to be derived at least in part from the sample media 

and holder. 

Fig. 9. EDS spectrum of silver particles collected near the 

low-volume furnace shows the lead and zinc have been 

removed but relatively low impurities of S, Fe, and Co remain. 

Note: This EDS spectrum is ‘uncorrected’ and the peaks from 

Cu, Si, and O must be assumed to be derived from the sample 

media and holder. 

Fig. 10 Light-scattering photometer data show particle mass 

concentration throughout the work shift on both floors. DISCUSSION

Filter samples

The results of filter sample analyses confirm that 

silver was the element of greatest concern. Airborne 

silver concentrations derived from all personal samples (ranging from 0.013 to 0.094 mg m -3 total silver) as well as the area filter samples from the 

furnace floor (ranging from 0.013 to 0.039 mg m -3

) 

exceeded the PEL for total silver, although none of 

the samples exceeded the ACGIH TLV of 0.1 mg 

m -3 for total silver. The concentrations measured 

for other elements were well below both the PELs 

and TLVs. The total silver concentrations from the 

area filter samples on the electro-refining floor were 

consistently lower than for the samples on the furnace floor and only one area sample from the 
electro-refining floor (collected on a work bench 

where cells were cleaned and maintained; Location 

N in Figs 4 and 5) exceeded the PEL and none 

exceeded the TLV. 

The two employees with the highest exposures to 

total silver (0.049 and 0.094 mg m -3

) were not on 

the furnace floor where the highest ambient concentrations were found, but rather on the electro-refining 

floor. This suggests that these individuals performed 

tasks that entailed a localized high exposure to silver 

that was not detected by the area samples or during 

the mapping of the electro-refining floor. This type 

of task-related exposure is also supported by the 

personal exposure of the furnace floor operator 

(0.038 mg m -3

) who also spent time in the chemistry 

laboratory. His exposure was the highest among the 

furnace floor employees, suggesting that he may 

have high exposure tasks in the chemistry laboratory 

or elsewhere in the refinery. The forklift worker on the 

furnace floor had exposures that were similar to the 

area samples collected from the furnace floor suggesting that the operation of the forklift does not increase 

the exposure to metal. The exposures of the furnace 

operators to total silver (0.018 and 0.029 mg m -3

) 

were also greater than the PEL, which suggests the 

local ventilation systems on the two furnaces may 

not be adequately capturing the fumes. 

The chemistry involved in electro-refining of the 

silver may be a potential source of soluble silver 

compounds, since soluble silver compounds have 

the potential to be mobilized via aerosolization of 

droplets from the liquid surfaces of the ion exchange 

cells. To investigate this, paired filter samples were 

collected above the electro-refining cells (Locations 

I–L in Figs 4 and 5). The first filter of each pair was 

analyzed for total silver, while the second was used 

to determine the soluble silver. Because of the low 

temperature/low agitation operating conditions of 

the cells, the generation of silver aerosols was apparently low, thus all paired area samples were below 

the PEL for total silver. Although some soluble silver 

was evidently mobilized, it was still significantly 

less than the ACGIH TLV. 

Table 2. Events depicted by photometer particle mass 

concentration 

Time Event 

10:00 AM Feedstock in high-volume furnace 

began melting and a forklift 

began operating 

10:50 AM Furnace operator using 

propane torches to warm 

the molds 

11:20 AM Begin anode mold pour 

12:15 PM Near end of anode mold pour. 

A forklift also drove by the 

sampling station 

1:30 PM Startup of low-volume furnace. 

Warming up molds using 

propane torch 

3:00 PM Forklift activity 

Aerosol mapping and characterization

The spatial maps of particle number concentration 

confirmed that the distribution of aerosols was initially low throughout the facility with a baseline 

level of particle concentration on the furnace floor 

in the range of 5 x 103

–5 x 104 particles cm -3 . 

TEM/EDS data indicated that the background particles were mainly carbonaceous, assumedly from 

fugitive fossil fuel emissions. 

The particle concentrations rose as the furnaces 

increased in temperature with a simultaneous decrease in the particle size (from a mean diameter 

of 100 nm for baseline particles to 

the furnace). Concentrations were highest 

10 nm near 

in the 

region around the furnaces during mold pours, representing up to a 1000-fold increase in particle number 

over the baseline levels. Concentrations .106 particles cm -3 were measured regularly and are similar 

or slightly higher than levels measured previously 

in a foundry setting (Evans et al., 2008). The high 

number of ‘nucleated metallic clusters’ (metal-rich 

nanoparticles with mean diameter of 10 nm) was 

often higher by a factor of two or three during the 

anode pours as compared to the pouring of ingots 

(Fig. 6), while both scenarios produced a ‘secondary 

mode’ of agglomerated nanoparticles with a mean 

diameter 25 nm. The TEM/EDS results indicated 

that most of these particles were low in carbon and 

high in metals, including silver and varying amounts 

of other metals such as lead, antimony, and zinc, suggesting they were derived primarily from the homogeneous nucleation of metal vapors. These particles 

differ considerably from those generated by combustion (mainly carbonaceous) or those resulting from 

metal vapors condensing onto existing particles, 

which result in more complex morphologies and 

composition (Miller et al., 2007). 

The baseline particle number concentration measured early in the day on the electro-refining floor 

was similar to the baseline on the furnace floor, but 

rose slowly over time. This may be a result of electro-refining activities or it may be the result of fugitive particles entering the electro-refining floor from 

the two furnaces below. The latter was supported by 

mapped data showing increased particle concentrations measured near the stairwells of the electrorefining floor relative to the rest of the room (Fig. 5) 
suggesting the upward diffusion or convection of particles, even though the local ventilation systems were 

operating on the furnace floor. 

This convection or diffusion of particles is further 

supported by the photometer data (Fig. 10). Work 

activities that produced increases in particle mass 

concentration on the furnace floor are sometimes reflected in the electro-refining floor data, with a short 

time lag. The best example of this occurred during 

the anode mold pour. The furnace floor photometer 

registered very high and quite erratic levels of airborne particulates during the molten metal pour, 

with peaks at 

refining floor 

11:20 and 11:55 AM. The electrophotometer measured similar peaks 

‘15 minutes later, although less erratic and dampened due to the effects of diffusion and convection. 

The FMPS data and TEM/EDS analysis of ESP 

samples collected during the pouring of molten silver 

confirm the generation of high numbers of metallic 

nanoparticles. The data show somewhat lower number concentrations during the pouring of refined silver 

(Fig. 3) than during the anode mold pour (Fig. 2), 

probably in part due to more effective ventilation 

around the low-volume furnace. The TEM/EDS data 

show that for both cases the particles are fairly uniform in morphology and have a proportionally high 

silver content, although the particles generated during 

the anode mold pour contained more contaminants 

than the refined silver particles. 

CONCLUSIONS

Worker exposure to airborne metals and nanoparticles was measured during refining of silver. The 

standard (filter sampling) approach provided measurements of worker exposure while the use of additional tools enhanced the results and provided a more 

complete picture of what was happening in the workplace. Enhancements included using real-time particle number concentration measurements along with 

subsequent spatial mapping to provide valuable information regarding particle generation and migration, while spot sampling and subsequent TEM/ 

EDS analysis provided additional information on 

the particle morphology and elemental composition 

of the aerosols. Taken together, a comprehensive exposure assessment was completed and provided useful information for workplace exposure control 

decisions. 

Results of the personal sampling revealed that all 

employee exposures exceeded the OSHA PEL for 

total silver, but not the ACGIH TLV. 

The particle number concentration maps indicated 

that even though the furnaces had ventilation controls in place, during the pouring of molten metal, 

airborne particle concentrations measured near the 

furnace were .1000 times higher than the baseline 

and FMPS data indicate a significant decrease in 

the mean particle size in that area. This suggests that 

the ventilation system was not capturing all the particles generated by the process and many were still 

traveling into the workplace from the furnaces. 

The TEM/EDS data confirmed the FMPS result 

and also showed that the particles near the furnace 

were high in metal content. This not only increases 

the mass of metals to which the furnace workers 

are exposed but the smaller particle size also increases the bioavailability and thus the health risk 

(Ji et al., 2007; Sung et al., 2008). In this case, lower 

exposure to workers could potentially be achieved 

through ventilation redesign and modified operation 

of the furnaces. 

To further investigate the potential for overexposures in such a workplace, time and motion studies 

could also be conducted, especially on electro-refining 

floor and chemistry lab personnel who had the highest exposures to silver. That analysis would help 

detail exactly where employees conduct various activities during a work shift. Combining that information with additional detailed mapping of particle 

concentrations would yield valuable insight into 

sources of worker exposure. Information from these 

alternative methods, as well as personal and ambient 

exposure data and real-time measurements (such as 

with a photometer), may help identify the most significant exposure sources where additional controls 

are needed. 

Acknowledgements—The authors wish to thank both Gib 

Ahlstrand from the University of Minnesota Biological 

Characterization Lab, who provided TEM/EDS analyses and 

manuscript suggestions, and Dale Marcy from North Idaho 

College who provided extensive manuscript review and 

editing. 

Disclaimer—The findings and conclusions in this paper are 

those of the authors and do not necessarily represent the views 

of the NIOSH. Mention of specific products and manufacturers 

does not imply endorsement by NIOSH. 

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