Particulate matter (PM) exposure and health effects have become top U.S. environmental research agenda items over the past decade. Environmental epidemiological studies rely on information from both sides of the dose-response equation: risk factor measures and health outcome data. The ability to resolve relationships from environmental data depends upon the quantity, accuracy, specificity, and precision of both. Although health surveillance and data collection methods have improved dramatically through database system advances, the techniques for PM exposure are not adequate.

Shown is a track of particles (blue streak) deposited on the quartz crystal microbalance of the device prototype. A thermophoretic heater located above the crystal substrate precipitates particulate matter from the air onto the microbalance.

At least three major limitations currently exist for PM measurement. First, PM exposure monitoring is too expensive. Currently, high capital costs make purchasing hundreds of samplers for a single study impractical. Overcoming this obstacle would be extremely beneficial to several types of studies, including measurements of community exposure, exposure variability between individuals in heterogeneous populations, and the relationships among indoor, outdoor, and total personal exposure levels.

Second, PM exposure assessment technology is limited by size, weight, noise, and power constraints. This applies to both area samplers (stationary monitors placed in commonly occupied spaces) and personal samplers (monitors worn by subjects at the breathing zone). The operating noise of area samplers can be very undesirable to study participants. Personal samplers are typically smaller than stationary samplers, but their pumps are usually loud, heavy, and bulky. These inconveniences alter subjects’ behavior and produce non-representative exposure estimates. Also, the most susceptible populations — children, the elderly, and those with respiratory ailments — often have limited tolerance for such samplers.

Third, simultaneous measurements of several PM characteristics by a single PM device are not typically available. As a result, several types of samplers must be used simultaneously to obtain PM mass, particle size distributions, chemical composition, and optical properties. A single multi-parameter sampler that could obtain this range of measurements would be much more practical, especially to identify the sources and mechanisms of health effects.

The apparatus described in this work is a miniaturized system for particle exposure assessment (MSPEA) for the quantitative measurement and qualitative identification of particulate content in gases. Unlike other PM monitoring systems, the true mass transducer simultaneously measures the mass, size distribution, and optical properties of particulate matter. The device has power requirements of less than 100 mW, and provides data consistent with the federal reference method for PM 2.5 and PM 10 analysis.

The invention utilizes a quartz crystal microbalance (QCM) or other mass-sensitive, temperature-compensated acoustic wave resonator for mass measurement. Generally, particles are deposited by thermophoresis onto a piezoelectric resonator surface. The resonator’s frequency is monitored. In one embodiment of the present invention, there is coupled to the QCM a size-selective inlet based on balancing the size-dependent gravimetric settling velocity of particles with the airflow rate through the collector.

The monitors can be as small as 2-3 cm3 if constructed without optical identification components. The MSPEA has been constructed from materials costing $100. A more sensitive prototype, the MEMS-PEA, has been manufactured using MEMS fabrication techniques. Because of the very low cost of materials, once scale-up has been achieved, this sensor-on-a-chip will cost significantly less to produce than the lowest-priced devices on the market today, which are more than two orders of magnitude larger. In some configurations, the MEMS-PEA device can measure a single picogram of material.

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