Terahertz radiation driven chiral edge currents in graphene.

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.

Dynamic Hall effect driven by circularly polarized light in a graphene layer.

We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates--under oblique incidence--an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed E and B fields which are, however rotating with the light's frequency.

Aligning chemical assessment tools across the hazard-risk continuum.

With the growing number and diversity of hazard and risk assessment algorithms, models, databases, and frameworks for chemicals and their applications, risk assessors and managers are challenged to select the appropriate tool for a given need or decision. Some decisions require relatively simple tools to evaluate chemical hazards (e.g., toxicity), such as labeling for safe occupational handling and transport of chemicals. Others require assessment tools that provide relative comparisons among chemical properties, such as selecting the optimum chemical for a particular use among a group of candidates. Still other needs warrant full risk characterization, coupling both hazard and exposure considerations. Examples of these include new chemical evaluations for commercialization, evaluations of existing chemicals for novel uses, and assessments of the adequacy of risk management provisions. Even well-validated tools can be inappropriately applied, with consequences as severe as misguided chemical management, compromised credibility of the tool and its developers and users, and squandered resources. This article describes seven discrete categories of tools based on their information content, function, and the type of outputs produced. It proposes a systematic framework to assist users in selecting hazard and risk assessment tools for given applications. This analysis illustrates the importance of careful selection of assessment tools to achieve responsible chemical assessment communication and sound risk management.

Harmonization of future needs for dermal exposure assessment and modeling: a workshop report.

Dermal exposure assessment and modeling is still in early phases of development. This article presents the results of a workshop organized to harmonize the future needs in this field. Methods for dermal exposure assessment either assess the mass of contaminant that is transferred to the skin, or the transfer of contaminant through the skin. Models for dermal exposure are either knowledge-based or deterministic. Any method or model should be transparent, validated, and open to further development. Some (partly) validated and standardized methods are available for measuring or modeling permeation of the skin or of personal protective equipment (PPE). Further validation and standardization is necessary. More research is needed on permeation of dusts and aerosols and more realistic tests should be developed and used for PPE. Several methods have been developed to measure contamination of surfaces or skin, but they are not validated or standardized. A number of non-validated models exist to assess dermal exposure. A clear need exists for more studies of dermal exposure, regarding measurement methods, models and actual exposure levels. A running four-year European study will greatly expand the knowledge in this field. Simple tools to assess and control the risks of dermal exposure in small and medium sized enterprises are also needed. Increasing the general knowledge of practitioners (e.g., safety professionals, occupational hygienists and physicians) in the field of dermal exposure is a first requirement. Available data, for example, on the permeation of PPE, should be made more readily available, using modern information technology. When information on dermal exposure is gathered and stored, the core information needs are partly the same as those for inhalation exposure. Some elements of process and activity, substance and product or worker, specific for dermal exposure, have been suggested by the workshop.

Evaluation of the mass balance model used by the Environmental Protection Agency for estimating inhalation exposure to new chemical substances.

The U.S. Environmental Protection Agency (EPA) is responsible for assessing the potential for unacceptable human health and environmental risks of new chemical substances prior to commercialization. Estimates of potential inhalation exposure to workers during manufacture, processing, and use of a new chemical substance are key elements of these assessments. However, the available information with which to assess the potential for exposure is often limited for new chemicals. One approach used by EPA to develop screening level estimates of inhalation exposure to vapors in the absence of data is the use of a mass balance model to predict the airborne concentration for various activities such as drumming and sampling. The mass balance model was evaluated by comparing the exposure estimates for specific operations with monitoring data reported in selected studies from the available literature. In general the estimated exposures based on the midpoint of the range of default input values were well within one order of magnitude of the measured exposures. Selection of more conservative (i.e., protective) model input values overestimated exposures by one or more orders of magnitude. There are many simplifying assumptions inherent in the model and many variables that influence exposure that are not considered. Uncertainty analyses of the model demonstrated that values selected for the ventilation flow rate and generation rate greatly influence the estimate of exposure and should be carefully chosen. Additional research is recommended, and ultimately, model validation should be completed to further improve and refine the model.