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

Re-entry into treated glasshouses during crop production (Scenarios 9-12)5.22 Dichlorvos residues on surface and in the air after indoor application areinfluenced by environmental parameters, such as the rate of air exchange,temperature and the amount of sunlight penetration. Deposited dichlorvos may bedegraded, adsorbed or re-enter the atmosphere (Brouwer et al., 1992 in APVMA,2008b). Rapid degradation can occur on concrete or glass, whereas wood may actas a reservoir (Hussey & Hughes, 1963 in APVMA, 2008b).5.23 The APVMA reported results from two studies carried out in a glasshouse mistedat 33 mg dichlorvos/m 3 , where the dissipation kinetics in the 2 experimentsshowed considerable variation, especially after the first 240 minutes postapplication(Brouwer et al., 1992 in APVMA, 2008b).5.24 In the first study the glasshouse was unventilated and there was a narrowtemperature range (11.6 – 14ºC), and gave the slowest dissipation. Airbornedichlorvos levels declined progressively from 8860 to 715 μg/m 3 over 6 hours.When regression analysis of Log 10 concentration vs time was performed, the Yintercept was 3.84 (or log [6900] μg/m 3 ) and the slope [the rate of loss ofdichlorvos] was -0.0031/min.5.25 In the second study the glasshouse was ventilated after 360 minutes. Airbornedichlorvos levels fell from 9910 to 875 μg/m 3 over 240 minutes. When regressionanalysis of Log 10 concentration vs time was performed, the Y intercept was 3.84(or log [6900] μg/m 3 ) and the slope was -0.0043/min. After 240 minutes thetemperature increased from 12 to 24ºC, and the dichlorvos concentrationincreased to 1000 μg/m 3 . The slope of the post-ventilation Log 10 concentration vstime plot was -0.018/min, faster than either of the pre-ventilation rate losses.REI for workers ventilating the glasshouse after treatment:5.26 As the New Zealand use rate in glasshouses is 50 mg dichlorvos/m 3 for crops (notflowers) the application rate will be approximately 1.5 times the rate used in theBrouwer et al. study. The regression plot intercept is adjusted upwards to 4.02(Log 10 [6900 x 1.515]) while the same slope (-0.0031/min) has been assumed.This is the approach and slope used by APVMA in their assessment forventilating the greenhouse (2008b).5.27 Taking the slowest initial decline in dichlorvos levels from the two studies, theairborne concentration of dichlorvos at 240 minutes will be:10 (4.02 + [-0.0031 x 240]) = 1888 μg/m 35.28 Re-entering the treated glasshouse to ventilate it would expose the worker byinhalation at approximately 1.89 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 XIII:Dichlorvos reassessment – application Page 315 of 436