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Radioactive gases and labelled particles have been used in wind tunnels to measure the effects of Reynolds and Schmidt numbers on transport to surfaces with widely spaced roughness elements. Good correlations are obtained for gases and for sub-micrometric sized particles, and the results can probably be extended to conditions in the open air. Deposition oflarger particles is dominated by the effects of bounce off, which depends on surface conditions. This is also reflected in the pattern of deposition between roughness elements.

To assess the contribution of lead in motor exhaust to the total uptake of lead it is necessary to know the amount deposited in the lungs, the uptake thence to blood and other tissues, and the rate of excretion from the body. There (Fig. 1). A similar effect was obtained by drawing the usual chain-aggregate type aerosol through a humidifier.In some experiments the container was exposed to sunlight for about 1 hr so that the aerosol received ultra-violet radiation before inhalation took place. InhalationThe subjects, who were seated at rest, inhaled the by copyright.

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Tetraethyl lead was labelled with 203 Pb, added to petrol, and burnt in an engine. The exhaust aerosol was inhaled by volunteers, usually after it had been stored in a box, but in a few experiments directly, a few metres downwind of the engine. The percentage deposition in the lung, uptake to blood and excretion of the labelled lead were measured. The percentage deposition in the lung depended on the length of the respiratory cycle. For a 4 s cycle (15 breaths/min) the deposition was 35 ± 2 (s. e.)%. Most of the 203 Pb was removed from the lung with a half life of about 6 h. About half of the amount deposited in the lung was in the blood at 50 h after inhalation. A similar fraction was in the blood at the same time after injection of 203 PbCl 2 in saline solution. From this and other evidence it is deduced that most of the 203 Pb passed from the lung to the blood plasma. About half was removed to bone and other tissues, and half became attached to red cells. From about 72 h onwards, the amount of 203 Pb in blood declined with a mean biological half life of 16 d. Assuming that the same half life applies beyond the period for which measurements were made, the results can be used to predict that con­tinuous (24 h/d) exposure to 1 μ g/m 3 of exhaust lead over a period of months will given an average contribution of about 1 μg/l00 ml to blood lead. Results of other work on experimental and industrial exposure of persons to lead in air appear consistent with this.