Behaviour of magnetic micro-particles in the human lung.

Magnetic micro-particles were used to investigate the defence system of the human lungs against foreign material. After primary magnetisation a remanent magnetic field (RMF) of the lung can be measured that allows estimation of the amount of dust retained in the lung. After calibration of the system with a lung phantom the magnetic contamination retained in the lungs of dental technicians and welders was estimated at mean values of 22 and 500 mg respectively. In normal controls only 0.3 mg was found. About 0.5 mg of spherical monodisperse magnetite particles was deposited in the alveolar region of the lung by voluntary inhalation. The decay of the RMF, called relaxation, results from a misalignment of the dipole particles due to the activity of pulmonary macrophages. This macrophage activity is characterised by a cellular energy Ez. With a secondary magnetisation the lung can be remagnetised by rotation of the dipole particles. This allows an estimation of the intracellular viscoelasticity and the motility of the alveolar macrophages in vivo. Secondary magnetisation and relaxation curves of spherical monodisperse magnetic particles are presented. Intracellular viscosity was estimated to be n approximately equal to 100 Pa.s at shear rates near 0.01 s-1, the rigidity modulus being v approximately equal to 4-8 Pa. Macrophage activity was described by a cellular energy EZ approximately 5 x 10(-18) J. Additionally, non-magnetic aerosol exposure resulted in a faster relaxation, which was interpreted to be due to activation of the macrophages. The magnetite particles were cleared with a half-time of approximately 110 days.