Potential Health Hazards from Thermal Degradation Events: Particulate vs. Gas Phase Effects

Accidental exposure to fumes arising from thermal degradation events is a major concern for manned space missions. Both particulate and gas phase components may be responsible for severe pulmonary damage occurring from such exposure. Results from our recent studies implicate ultrafine particles (particle diameter in the nm range) as potentially severe pulmonary toxicants. We have conducted a number of in vivo (inhalation and intratracheal instillation studies in rats) and in vitro studies to test the hypothesis that ultrafine particles elicit significantly greater adverse pulmonary effects than larger-sized particles. We used as surrogate particles ultrafine TiO₂ particles (12 and 20 nm diameter) and carbon black (20 nm diameter). Results in exposed rats showed that the ultrafine TiO₂ particles not only induce a greater acute inflammatory reaction in the lower respiratory tract than larger-sized TiO₂ (250 nm diameter) particles, but can also lead to persistent chronic effects in the deep lung as indicated by an adverse effect on alveolar macrophage mediated clearance function of particles and beginning interstitial fibrotic lesions. An increased access of the ultrafine particles to the pulmonary interstitium is one factor for their pulmonary toxicity. In vitro studies with lung cells (alveolar macrophages) showed in addition that ultrafine TiO₂ particles have a greater potential than larger-sized TiO₂ particles to induce mediators which can adversely affect other lung cells. Other studies in rats with inhaled surrogate gas phase components, such as HCI and HF showed that these compounds injure only the upper respiratory tract, in contrast to the ultrafine particles. We conclude from our present studies that ultrafine particles have a significant potential to injure the lung and that their occurrence in thermal degradation events can play major role in the highly acute toxicity of fumes. Future studies are planned with adsorption of typical gas phase components (HCI) on the particles to investigate combined effects and to differentiate between gas and particle phase effects and to perform mechanistic studies aimed at introducing therapeutic/preventive countermeasures. Although countermeasures may be helpful, the classic occupational approach using engineering controls to reduce and/or prevent exposure should remain the primary focus of control.