Comparing pollen and spore counts collected with the Rotorod Sampler and Burkard spore trap.

BACKGROUND: The Rotorod Sampler and Burkard spore trap are air-sampling instruments commonly used by allergists in the United States. Although both devices are volumetric, their principles of operation and particle recoveries differ.
OBJECTIVE: This review will develop some guidelines for interpreting and comparing pollen counts obtained with these instruments. DATA SOURCES: Investigations examining particle recovery by each device will be reviewed. Five studies where the Rotorod and Burkard were operated in parallel will also be assessed.
RESULTS: The Rotorod's theoretical and empirical collection efficiencies are low for particles <10 microm but typically exceed 80% for particles above this threshold. This instrument has traditionally been considered insensitive to wind; experimental data present a mixed picture. The Burkard offers high collection efficiencies, particularly for small particles, when an aerosol's velocity is low. Bi-directional errors in collection efficiency occur as a function of increasing wind speed and particle size. Parallel trapping investigations demonstrated that the Burkard yielded a higher estimate of the atmospheric particle concentration for all particle sizes. Differences were widest for small fungus spores but narrowed for pollen-sized particles. Some recovery differences are readily explained by sampling theory. Other disparities may reflect over-sampling, under-sampling or each device's principles of operation.
CONCLUSIONS: Both instruments appear to record the same relative changes in airborne particle concentrations. The Burkard appears to be a superior instrument for sampling particles <10 microm. The Rotorod appears to be equal or superior to the Burkard for collecting particles >10 microm. A rough empirical means for comparing differences in particle recovery is presented.