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Characterization of Indoor Fine Particulate Matter Sources
Assessment of human exposure to fine particulate matter (PM) is not complete without considering the indoor environment, given the fact that most Americans spend nearly 90% of their time indoors. This project is aimed at identifying and characterizing the major indoor sources for fine PM. We are currently working in three areas: fine PM emissions from unvented heaters, lead emissions from lead-wick candles, and fine PM emissions from incense burning. Major findings are discussed below.
Although unvented space heaters have been banned in several states and foreign countries, they remain popular in many parts of the United States. We tested six unvented heaters in the test house in 2001, including two kerosene heaters, one natural gas heater, and three propane heaters. The major purpose was to determine their fine PM emission rates. One bedroom was isolated from the rest of the house and used as a test chamber. Particle-free air was introduced into the test room through an in-line fan, which maintained a slightly positive pressure to keep ambient particles from entering the room. The ventilation rate was 1 to 1.5 air exchanges per hour. The mass concentrations of PM2.5 and PM10 were measured gravimetrically. Particle size distribution was monitored with an electrical low-pressure impactor. Gaseous co-pollutants (CO, SO2, and NOx) were also monitored. Results suggest that kerosene heaters emit more PM and gaseous pollutants than the other two types of heaters. In three kerosene heater tests, the indoor PM concentrations ranged from 80 to 160 µg/m3. Most particle mass was in the fine range. Ion chromatographic analyses showed high levels of sulfate in the particles from this source. Based on 14 filter samples (2 outliers excluded), the particle-bound sulfate contents ranged from 52 to 78% (by weight) with an average of 68%. According to the literature, sulfate-containing particles in the respirable range have been associated with increased PM toxicity. In addition, the sulfate ion is often found as sulfuric acid, with increased adverse effects in asthmatics.
The candle-purchasing public is generally not aware that the core of candle wicks may contain lead. Used as a stiffening agent to keep the wick out of the molten wax, lead can be emitted as particulate to the air and then deposited on indoor surfaces. To define the problem we purchased locally 100 sets of candles that appeared to have metal-core wicks and found that 8% contained lead wicks. Candle samples are shown in Figure 1. We burned the candles to completion in a flow-through chamber to capture the air emissions, as shown in Figure 2, and extracted the candle residue to close the lead mass balance. Filters are shown in Figures 3 and 4. We found that individual candles emitted air lead at average rates that ranged from 100 to 1700 µg/hr. Assuming realistic indoor conditions, we modeled these emission rates to develop room concentration, child inhalation, and indoor deposition scenarios. Burning single candles can easily raise the source room concentration above Clean Air Act ambient lead concentration regulatory limits of 1.5 g/m3. Burning multiple candles can elevate it above OSHA-permissible exposure limits of 50 µg/m3.
Figure1. Candles containing lead in the wick or pigment
Figure 2. Experimental setup for capturing emissions from candles
Figure 3. Each filter represents 3 hours of emissions from a pyramid candle
Figure 4. Each filter represents 3 to 6 hours of emissions from a tall pillar candle
Burning incense emits fine PM in large quantities compared to other indoor sources. We measured PM emissions for 23 different types of incense using a cyclone/filter method. Emission rates for PM2.5 ranged from 7 to 202 mg/hr, and PM2.5 emission factors ranged from 5 to 56 mg/g of incense burned. Size distributions by mass typically ranged from approximately 0.06 to 2.5 µm in aerodynamic diameter, with peak values between 0.26 and 0.65 µm. An indoor air quality model showed that burning incense at home can cause indoor concentrations of PM2.5 to far exceed the NAAQS (24-h average) for fine PM.
Testing of indoor combustion sources is nearly completed, as of the first quarter of 2002, and the focus is now shifting to non-combustion sources. Sources under consideration include household products that may generate liquid aerosols (e.g, aerosol cans) and office equipment (e.g., printers and photocopiers).
Contact: Zhishi Guo, 919-541-0185, guo.zhishi@epa.gov