262 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 7 Estimated Respirable Percent of Total 

 Airborne ^^^Pu Flux at Rocky Flats 



(respirable) {\}.J\J) + nonrespirable 



where respirable = (d/min) min~' collected in impactor 

 nonrespirable - (d/min) min"' collected in cowl 



Ug = average flow rate through cowl inlet, 



0.36 m/sec 

 Ua - average wind speed, 0.9 m/sec 



and 1.1 -/nm diameter (which are impactor-stage 50% cutoff diameters for unit-density 

 spheres) as well as smaller particles collected on the impactor backup filter. From the 

 inhalation standpoint, particles collected on the l-jim stage should not be included within 

 the respirable particle size range. Only the smaller particles are usually considered 

 respirable. However, in the present comparison between cowl-collected nonrespirable 

 particles and impactor-collected respirable particles, there is much uncertainty in 

 calculating the relative 1-^xm particle concentration as compared with the cowl-collected 

 particle concentration. 



The better estimate of the respirable plutonium fraction at Rocky Flats is shown in 

 the last column of Table 7. On the basis of the assumed correction factor, the fraction of 

 respirable airborne plutonium changed from 4 to 98% for an isokinetic sampling 

 assumption to 9 to 99% of the total airborne plutonium. True fractions of respirable 

 plutonium should be between these limiting values. 



Fractions of respirable plutonium have been reported (Volchok, Knuth, and 

 Klemman, 1972) as 25% for plutonium collected wdthin particle cascade impactors at 

 Rocky Flats. Since the present results show that plutonium is also attached to particles in 

 much larger size ranges for cowl-collected samples than for particles collected on the 

 intial stage of an impactor, published fractions of respirable plutonium are probably 

 indicative of maximum fractions at each sampling location rather than a true fraction. 



Even if all plutonium collected in the cowl-impactor sampling systems were 

 respirable, airborne plutonium concentrations were still below the MFC (International 

 Commission on Radiological Protection, 1959). The MPC168 hr-air for ^^^Pu is 

 6 X 10~'^ juCi/cm^. The smallest fraction of respirable plutonium in Table 7 is 3.8%. 

 This sample at 0.3 m for site A also represents die largest airborne plutonium 

 concentration measured in our experiments. For this sample the real respirable ■^ ^ ^ Pu 

 concentration (<3.3 pm diameter) was 2 X 10"'^ /iCi/cm^, whereas the concentration 

 was 5 X 10"^^ ]uCi/cm^ if the l-^im particles of the particle cascade impactor were 



