Research Programs: Particulate Matter (PM) Health Effects
Research Areas: Dosimetry and Fate

PM Health Effects

What are the deposition patterns and fate of PM in the respiratory tract of susceptible people?

For health risk assessments, the most relevant measure of exposure to PM is the actual dose of particles deposited in the lung. Dosimetry information is important to understanding how a complex mixture of ambient PM may cause adverse health effects and may help explain why some people are more susceptible to the adverse effects of PM than others.

EPA has developed a new, non-invasive method, the bolus method, to measure the deposition of inhaled particles in different regions of the human lung. Studies using this method have significantly advanced our understanding of how different size classes of PM are deposited and revealed important information about the influence of age, gender, and respiratory diseases on the distribution of particle deposition within the lung.

Effects of Age and Gender

EPA researchers have found that the respiratory dose is similar between young and older adults; therefore, dose itself may not be responsible for differences in susceptibility to the health effects of PM by age. Similarly, the overall respiratory dose is comparable between men and women; but, in general, women tend to receive greater doses in their upper airway regions than men, possibly because their airways are somewhat narrower. This is consistent with some data that suggest women are more sensitive to irritant gases.

Effects of Particle Size

The hypothesis that ultrafine PM (particles with diameters less than 0.1 µm) may be responsible for the association of health effects with ambient PM prompted EPA investigators to evaluate the effect of PM size on deposition efficiency. Surprisingly, the ultrafine and coarse PM fractions are deposited in the same areas in a healthy human respiratory tract. While some particles make their way into the deep lung, the majority appear to be removed higher in the respiratory tree. In contrast, a small study conducted by EPA's research partners showed that the total pulmonary deposition of inhaled ultrafine particles was higher than that of fine particles in healthy individuals and was also higher among subjects with asthma and during exercise. Researchers have also studied the influence on size on the deposition of fine and coarse particles by comparing deposition in the nasal cavity of anesthetized rats and to particle deposition in a nasal mold. This work demonstrated that deposition fraction varied by flow rate, direction, and particle size. Efforts using experimental data and physical concepts to develop models of particle deposition as a function of age are currently under development. It is clear, however, from preliminary data that the dose of PM actually deposited is different for different size fractions and that deposition depends largely on the size distributions of exposure aerosols.

Effects of Impaired Lung Function

Fractional Deposition of 1 µm

Fraction of inhaled small particles deposited in the lungs of human subjects. Those with various forms of airway and lung disease have a higher rate of deposition than healthy subjects.1

While PM dosimetry in adult, healthy, human males has been well described, more work needs to be done to understand dosimetry in impaired lungs. The respiratory dose of inhaled PM is unevenly distributed within the lung, and the actual dose in local airway regions can be many times greater than the average lung dose. People with impaired lungs may experience doses to airway "hot spots" that exceed those in a similar location of a healthy lung by a factor of 8 or 10. Therefore, exposure relationships alone may be inadequate to address responsiveness in potentially susceptible groups without greater detailed assessments of the dosimetry.

Translocation of PM After Deposition in the Lung

In light of recent findings concerning the cardiac and systemic effects of PM, researchers have attempted to address the movement of PM out of the lung into other systems after deposition. Preliminary data suggest that some PM may, as a function of size and composition, migrate from the lung to other organs and tissues. Studies using animals have shown that that size alone does not dictate the fate of particles; rather, their transport appears to depend to a greater extent on particles' composition. Although these data need further confirmation, their implications are significant, especially in light of the cardiac effects attributed to PM. Research in this area is ongoing.

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