D iscussion Sources of Lead in Perennial Ryegrass and Radishes John H. Harley Director, Health and Safety Laboratory, US.Atomic Energy Commission, New York, N.Y. he experiment that has been described has been carefully planned and analyzed. Thus it is very difficult to criticize the work or to add substantially to the details given. I will therefore have to look at their peripheral conclusions and perhaps give a sketchy extension of the problem. It does seem that the authors have presented considerable proof that the lead in the root and aerial parts of the plant derives from soil as well as fallout from the atmosphere. Of course, the amount of lead in the soil which has been recently derived from atmospheric sources may be a factor, since this form of lead may actually be more available to the plants. I will now try to summarize some of the data from our own laboratory and from others which give a picture of the human intake of lead. For example, Table I shows the concentration of stable lead in surface air along our 80th Meridian Network. Obviously New York City is the highest location sampled. Even in this case, if we use 2 kg./m.3 as an average concentration and assume a breathing rate of 20 m.3 a day, the total annual intake by man would only be about 15 mg. The usual values for drinking water run about 10 pg. of lead per liter. The standard consumption of 400 to 500 liters per year would thus contribute 4 or 5 mg. of lead per year. Table I1 shows the amount of lead found in various diet components in New York City and our estimate of total lead intake, using Department of Agriculture consumption statistics. It is very apparent that the majority of man's exposure to lead comes from dietary sources. It may well be that the availability of lead for metabolism may be different for air, water, and food, but in any case it is probable that the diet is the largest contributor to our body burden of lead. If we now examine the authors' point that prehistoric man
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Table I. Lead Concentrations in Surface Air (1967) Latitude kg. per m 3 Thule 70' N
