Evaluation of hydrogen peroxide and ozone residue levels on N95 masks following chemical decontamination.

BACKGROUND: Hydrogen peroxide and ozone have been used as chemical decontamination agents for N95 masks during supply shortages. If left behind on the masks, the residues of both chemicals represent a potential health hazard by skin contact and respiratory exposure. AIM: Characterization of hydrogen peroxide and ozone residues on mask surfaces after chemical decontamination. METHODS: Various N95 masks were decontaminated using two commercial systems employing either aerosol spray or vaporization of hydrogen peroxide in the presence of ozone. Following the decontamination, the masks were aired out to eliminate moisture and potential chemical residues. The residual hydrogen peroxide and ozone were monitored in the gas phase above the mask surface, and hydrogen peroxide residue directly on mask surfaces using a colorimetric assay. FINDINGS: After decontamination, hydrogen peroxide and ozone were detectable in the gas phase in the vicinity of masks even after 5 h of aeration. Hydrogen peroxide was also detected on all studied masks, and levels up to 56 mg per mask were observed after 0.5 h of aeration. All residues gradually decreased with aeration, likely due to decomposition and vaporization. CONCLUSION: Hydrogen peroxide and ozone were present on N95 masks after decontamination. With appropriate aeration, the gaseous residue levels in the vicinity of the masks decreased to permissible levels as defined by the US Occupational Safety and Health Administration. Reliable assays to monitor these residues are necessary to ensure the safety of the mask users.

The sliding slit test for dynamic IMRT: a useful tool for adjustment of MLC related parameters.

For treatments with dynamic intensity modulated radiotherapy (IMRT), the adjustment of multileaf collimator (MLC) parameters affecting both the optimization algorithm and dose distributions is crucial. The main parameters characterizing the MLC are the transmission (T) and the dosimetric leaf separation (DLS). The aim of this study is twofold: a methodology based on the 'sliding slit' test is proposed to determine (T, DLS) combinations inducing the best conformity between calculations and measurements. Secondly, the effects of the MLC adjustment on measured dose and on optimization are presented for different configurations as the chair test and for the patient dosimetric quality control (DQC). Tests were performed with a Varian 23EX linac operated at 20 MV and equipped with a 120 leaf Millenium dynamic collimator. The treatment planning system was CadPlan/Helios (version 6.3.6). Results demonstrated that the sliding width (SW) strongly depends on the (T, DLS) combinations, and the measured dose is a linear function of the SW. Different (T, DLS) combinations induced a good agreement between calculations and measurements. The influence of the MLC calibration was found to be particularly important on the 'sliding slit' test (11.8% for a gap change of 0.8 mm) but not so much on the chair test and on the DQC. To detect small variations in leaf adjustment and to ensure consistency between calculation and actual dose delivered to patients, a daily check called IMRT MU check is proposed.

Biosphere atmosphere exchanges: Ozone and aerosol dry deposition velocities over a pine forest.

Measurements of dry deposition velocities for ozone and aerosol particles has been measured over a pine forest in the south-west of France by the eddy correlation method. Low frequency fluctuations in the variations of scalars can introduce erroneous values of vertical fluxes for ozone and aerosol particles but also, in some cases of low wind, for momentum and heat. The dry deposition velocity variations are similar for ozone and aerosol, although the mechanism of deposition are different. In the case of aerosol particles a parametrization, not far from that of Wesely et al. (1985) can be given.

[Development of the rank-size relationship in the Quebec urban system between 1871 and 1976]. / Evolution de la relation rang-taille dans le systeme urbain quebecois entre 1871 et 1976

PIP: The author uses census data to analyze 10 variables relating to the distribution of city size in the province of Quebec between 1871 and 1976. Three phases in the development of the urban system are identified: the period from 1871 to 1921, when urban primacy reached a maximum; the transition-maturation period from 1921 to 1956, when lognormality reached a maximum; and the period from 1956 to 1976, when the metropolitanization process resulted in a renewed trend toward urban primacy. The ethno-linguistic dualism existing in Quebec is discussed as a possible factor contributing to the unbalanced nature of the urban structure.^ieng

[Urban exodus, an attempt at a classification of the ex-urban population of the Eastern Townships]. / L'exode urbain essai de classification de la population exurbaine des Cantons de l'Est

PIP: The author presents an analysis of the results of a survey of 280 former urban residents now living in the Eastern Townships area of Quebec (SUMMARY IN ENG)^ieng