Application for the Reassessment of a Hazardous Substance under ...

Re-entry into treated enclosed industrial spaces (Scenarios 17-23)5.67 The APVMA used a study by Schofield (1993; Original not sighted) for data ondichlorvos behaviour after fogging industrial buildings (APVMA, 2008b). Thestudy was carried out in a large industrial building fogged to a targetconcentration of 85 mg/m 3 . The airborne dichlorvos levels were measured for 6hours, and then the building was ventilated for one hour before the workforce reentered.The half-life of airborne dichlorvos over the first dissipation phase was1.6 hours. The author‘s regression analysis of L n concentration vs time gave theY intercept at 2.996 (or ln [20] mg/m 3 ) and the slope [the rate of loss ofdichlorvos] was -0.4207/hour. [Schofield‘s analysis expressed the result innatural logarithms.] Schofield‘s data indicated that the dissipation half-life ofdichlorvos after ventilation slowed to 12.7 hours. [TCL notes the finding isunexpected.] The regression analysis of L n concentration vs time gave the slope[the rate of loss of dichlorvos] as -0.0544/hour. The R 2 of the second phaseanalysis was 0.74 (first phase, 0.98) indicating considerably more variability anduncertainty in the latter values, which may explain the finding.5.68 The APVMA (2008b) noted that, based on Schofield‘s data, after an applicationof RTU gas at the Australian label rate of 33 mg/m 3 into industrial buildings atoxicologically unacceptable level of dichlorvos would persist for 3 days, but aREI of 4 days could be supportable. However, as the New Zealand use rate is 50mg/m 3 , even a 4-day REI may not be supported. Risks for the New Zealandapplication rate are estimated further below (see Tables XVIII & XIX).5.69 It should be noted that product labels (Nuvos, P001132; Divap, P006080)recommend that treated enclosed spaces should be kept closed for 10-12 hoursbefore ventilation, and another 10-12 hours should elapse before recommencingwork in the buildings.REI for workers ventilating industrial buildings after treatment at 50 mg/m 3 :5.70 As the main New Zealand use rate is 50 mg dichlorvos/m 3 for industrial buildingsthe application rate will be approximately 59% the rate used in the Schofieldstudy. The regression plot intercept is adjusted downwards to 2.465 (L n [20 x0.588]) while the same slope (-0.4207/h) has been assumed. [Schofield‘s analysisexpressed the result in natural logarithms.]5.71 Taking the initial decline in dichlorvos levels, the airborne concentration ofdichlorvos at 10 hours will be:e (2.465 + [-0.4207 x 10]) = 0.175 mg/m 35.72 Re-entering the treated building to ventilate it would expose the worker byinhalation at approximately 0.175 mg/m 3 . If the duration of exposure was 30minutes, the worst-case upper limit, the inhalation exposures with various RPEwould be the values set out in Table XVIII:Dichlorvos reassessment – application Page 327 of 436