In the 1970s, lead was identified as a public health hazard, linked to neurological disorders in children and cardiovascular effects in adults. Infants and young children were found to be especially sensitive to low levels of lead, which was linked to behavioral problems, learning deficits and lowered IQ. With the establishment of the Clean Air Act, the EPA was required to set National Ambient Air Quality Standards (NAAQS) for lead, as well as five other criteria pollutants. In 1978, the standard for lead was set at 1.5 µg/m^³.

The federal government, including the EPA, took steps to regulate the propagation of lead by phasing out leaded gasoline, and creating standards for lead in paint, drinking water, and solid waste. Since then, the ambient lead levels have fallen 94 percent and the concentration of lead in children's blood has fallen to almost a tenth of what it was in the 70s.

However, it is estimated that 1,300 tons per year (tpy) are still being emitted into the air from approximately 16,000 sources, such as aviation fuel, metals and manufacturing industries, and incinerators/boilers.^[4]

Regulations

High volume samplers are typically located on elevated platforms.

During recent NAAQS reviews conducted by the EPA, and with new information from the health professional community, the agency revised the standard for lead down to 0.15 µg/m³, one tenth of the previous standard, calculated as a rolling three-month average of total suspended particulates (TSP). This primary standard will be used to set limits to protect public health from the adverse effects of airborne lead, while the secondary standard will set limits to protect public welfare, such as damage to crops, animals and buildings. The secondary standard will be the same as the primary standard.^[1]

The 2008 rule also requires states to monitor areas near point sources that emit 1.0 tpy or more, as well as 101 urban areas with a population of 500,000 or more. A point source is considered to be a single, identifiable source of lead emissions. The new regulations will mean that 135 emission sources will now fall under the new guidelines. The EPA estimates that 236 new or relocated monitors will be required to comply with the new regulations, all of which must be operational by Jan. 1, 2011.^[3] These additional monitoring sites will help identify sources that violate the lowered NAAQS standard.

In December 2009, the agency proposed further changes to the NAAQS that would:

• Lower the lead emissions monitoring threshold for point sources themselves (lead-emitting industries) from 5.0 tpy to 0.5 tpy.^[1] This is based on EPA estimates that 1,300 point sources in the National Emissions Inventory emit at least 0.1 tpy, accounting for 94 percent of all the lead emitted from source emissions, accounting for 1,058 tpy.
• Require lead monitoring at all NCore Network sites, which all states are required to begin operating by Jan. 1, 2011. A total of 80 such state-run sites are multi-pollutant, trace-level sites with meteorological instrumentation that are intended to evaluate ambient air quality attainment. They also provide data that is used to better understand pollutant interaction, air quality conditions and pollution modeling.
• Require lead monitoring at airports that accommodate piston-engine aircraft. Such airports were considered point sources by the EPA in their Air Quality Assessment study since they still use lead-containing fuel. The volume of leaded aviation fuel supplied in the United States in 2006 was 280 million gallons, the majority of which contained 2.12 grams per gallon, amounting to 623 tons, or 45 percent of the national inventory.^[2]

Monitoring techniques

New high-volume samplers, such as this one from American Ecotech, can adjust the flow automatically to correct for filter loading and ambient conditions.

In order to properly assess the sources and potential risks of airborne lead, several factors must be considered, including the proximity of the sampler to source emissions and populations, current meteorological conditions, background conditions and promulgation interferences to name a few.

Current technology is not capable of real-time monitoring for lead, so traditional high-volume samplers must be used. Many improvements have been made to the original samplers since the high-volume lead monitoring programs began in the 1970s. Typical lead samplers draw ambient air through a TSP size-selective inlet and the particulates are captured on an appropriate filter measuring 8 by 10 inches.

Normally, these samplers are run for 24 hours every six days, but in some non-attainment areas, samplers may be required to run every three days. Other size-selective inlets are available, such a PM10 and PM2.5, but they are unsuitable for lead monitoring since the cut sizes are insufficient for larger lead particles, especially near sources that may emit fugitive dust, such as materials handling facilities, crushers, stockpiles, etc. Low-volume samplers are also available, and typically draw ambient air through a 47 mm filter at 1 m³/hr. However, Federal Reference Methods (FRM) and Federal Equivalent Methods (FEM) currently are based solely on high-volume samples.

Lead concentrations are determined based on the mass of particulates collected on the filter, the volume of air sampled, and length of sampling time. High-volume samplers draw ambient air through the filter at flow rates of 70 to 100 m³/hour, however, older samplers are unable to actively control the sample flow, and therefore, as the filter increasingly becomes loaded with particulates, the flow rate can decrease. A large enough decrease in flow can affect the calculated lead concentrations on samplers where a constant flow is assumed. Using a sampler with active volumetric flow control will correct for this. These samplers keep a constant flow through the filter by increasing the pump's flow rate based on filter loading. Using ambient temperature and pressure readings to adjust flow rates is another benefit of such samplers. This is especially important when sampling is undertaken in areas of extreme altitudes and temperatures, as well as areas that may experience drastic weather changes.

Great care must be taken when handling the filters before and after sampling to avoid dislodging collected particulates or introducing foreign matter from outside the sampling method. For lead monitoring, this is especially critical since the TSP inlet allows particles of very large sizes to pass onto the filter. Since it is common that several days may pass since the sample was taken, or before the sampler is activated, strong winds could transport foreign matter onto the filter even with the sampler off, or for that matter blow sample particles off the filter. For this reason, some samplers are equipped with passive loading shields which are essentially automated shields that protect the filter from non-sampling particles.

Once a sample is collected, the filter is returned to a laboratory where it is weighed and analyzed for lead concentrations. Several methods exist for the analysis of lead, including X-Ray Fluorescence (XRF), Atomic Absorption (AA) and Inductively Coupled Plasma Mass Spectrometry (ICPMS).

Other benefits of modern samplers, besides active volumetric flow control and temperature/pressure correction, include internal data logging of flows, temperatures and pressures, as well as the ability to connect external meteorological instrumentation, such as wind-speed and direction sensors. Samplers with the capability of being triggered externally allow sampling of air from specific directions, or only during dry conditions. External triggers are typically meteorological instruments such as tipping-bucket rain gauges or wind-direction sensors.

Regular maintenance is essential for any air quality monitoring instrumentation. For high-volume samplers, regular cleaning is a must. Rogue particulate matter can collect on the inlet and fall onto the filter, adversely affecting the accuracy of the sample. Maintaining proper calibration of the temperature and pressure sensors is also necessary to assure accurate flow rates. For samplers with active flow control, the flow rate should also be audited periodically, and calibrated when needed. Routine preventative maintenance can keep your analyzer running flawlessly for years, while waiting until something goes wrong will only ensure that you spend your time putting out fires, which ends up costing more money in the long run.

Conclusion

The recent changes in federal regulations will impact many states and many industries, causing the majority of facilities to reassess their current monitoring programs to ensure their compliance. Careful planning and research will produce valid, defensible results, while improvements in technology have increased the effectiveness and accuracy of modern high-volume samplers. No longer the simple vacuum machines of the 1970s, today's samplers can save time and money, providing useful data that will help reduce our exposure to harmful pollutants and ensure cleaner air for our future. PE