¹ From the Biology Division, Argonne National Laboratory, Chicago

1. Mice were continuously exposed to air containing C¹⁴O₂. The specific radioactivities of urea carbon, total fecal carbon, and numerous components of tissue carbon were compared as a function of the duration of exposure with the radioactivity of the air CO₂.

2. The data indicate that the total CO₂ fixed from the air is proportional to its concentration in the air.

3. When the CO₂ concentration in the air is normal (0.03 per cent) about 0.34 per cent of the carbon of urea originates from air CO₂. A lesser proportion of the non-urea carbon of urine has its origin from air CO₂.

4. Only about 0.0054 per cent of the total fecal carbon is derived from air when the CO₂ concentration is 0.03 per cent. The constituents, which are extractable with alcohol and water, contain considerably higher proportions of fixed carbon than either the insoluble residue or the ether-extractable material.

5. The rates of uptake at the beginning of the exposure and the rates of loss at the termination of the exposure differed strikingly among the tissues studied.

6. However, the ultimate ratio of fixed air CO₂ carbon to total carbon in these tissues seemed to be approaching limiting values which would not vary by more than a factor of 3 from one another. It appears that of the total organic carbon in an adult mouse approximately 0.01 per cent may originate directly from the CO₂ of the air when the animal respires in air having a CO₂ concentration of 0.03 per cent, and that 1.8 per cent or more of the total carbon may originate from CO₂ within the animal.

7. Data are presented which indicate the life span of the mouse erythrocyte to be 49 days.

8. Calculations made on the basis of these experimental data and the accepted standard for permissible radiation in the human would allow mice to be continuously exposed to air containing 31 µc. per c.m. without ever exceeding the accepted permissible level for humans.

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