The World Health Organization (WHO) has a commitment to helping Member States achieve safe, sustainable and health-enhancing human environments, protected from biological, chemical and physical agents. The latter includes advising on the health impact of electromagnetic fields (EMFs) and radiation.
The results of the WHO/ICNIRP/NRPB workshop on static magnetic fields, published in this volume, provide a valuable and much needed contribution to the health risk assessment of exposure to static electric and magnetic fields, which is currently being coordinated by the WHO's International EMF Project. This WHO health risk assessment will be published as an environmental health criteria (EHC) monograph in early 2005.
This paper briefly gives an overview of the process of developing the WHO static fields EHC monograph, the criteria applied to studies that could contribute to the EHC, along with the ‘weight-of-evidence’ approach to health risk assessment. In addition, there is an increasing awareness of the need to account for uncertainty in the science database. This is traditionally addressed by further research, and the EMF project addresses these needs through the development of a ‘research agenda’. However, research programmes may take several years to complete, and the long latency associated with diseases such as cancer in people may also preclude a rapid outcome in some studies. The issue of current uncertainty is being addressed by the WHO EMF project through the development of a ‘precautionary framework’ in which precautionary measures will be applied to policy recommendations. 相似文献
The resolution value claimed for an electron microscopical three-dimensional reconstruction indicates the overall quality of the experiment. The Fourier shell correlation (FSC) criterion has now become the standard quality measure. However, what has continued to be controversial is the issue of the FSC threshold level at which one defines the reproducible resolution. Here, we discuss the theoretical behaviour of the FSC in conjunction with the various factors which influence it: the number of "voxels" in a given Fourier shell, the symmetry of the structure, and the size of the structure within the reconstruction volume. Both the theoretical considerations and our model experiments show that fixed-valued FSC threshold (like "0.5") may never be used in a reproducible criterion. Fixed threshold values are-as we show here-simply the result of incorrect assumptions in the basic statistics. Two families of FSC threshold curves are discussed: the sigma-factor curves and the new family of bit-based information threshold curves. Whereas sigma-factor curves indicate the resolution level at which one has collected information significantly above the noise level, the information curves indicate the resolution level at which enough information has been collected for interpretation. 相似文献
Type-specific reference conditions that describe sites with no or only very minor anthropogenic disturbances are a basic requirement
of the European Water Framework Directive. The reference condition approach implies a previous determination of criteria acceptable
for the definition of near-natural stretches. In this paper, a methodology based on selection and validation procedures is
applied to a Portuguese watershed to guide the identification of reference sites. The methodology consisted of three phases:
(1) a preliminary site inspection through the use of maps, available data and an extensive screening field campaign that resulted
in the selection of 52 potentially undisturbed small- and medium-sized rivers stretches, ranging from 43 to 1069 m in altitude,
with catchment areas between 4 and 641 km2 and representing siliceous and calcareous riverbeds; (2) a detailed site selection based on ten pre-defined criteria involving
physical/social/biological attributes that indicated catchment land use changes and alien vegetal riparian species as major
human impacts; (3) a site validation procedure involving detailed investigation of benthic macroinvertebrate communities,
riparian vegetation, in-stream habitat quality and chemical parameters that showed riparian wood-related problems as a major
restriction to validate a stretch as a near-natural site. Only about 12% of all investigated river sites, accounting for 600 m
of all 5200 m studied, could be considered as near-reference stretches or sites retaining essential natural functions. Selection
and validation procedures can identify different sites as reference, which reinforces the need for applying both procedures.
The results presented can help to accomplish the requirements of the EU – Water Framework Directive by selecting reference
sites as the first step to establish biological reference conditions and, simultaneously, to form a basis for nature conservation
strategies.
Electronic supplementary material Electronic supplementary material is available for this article at
and accessible for authorised users. 相似文献
Identifying releases from suppression represents one of the most fundamental dendroecological procedures for quantifying forest disturbance histories. In this study we evaluate boundary-line release criteria, which incorporates the effects of growth history on release response. In eastern hemlock (Tsuga canadensis L.) the maximum possible value of a pulse in percent-growth change is dependent on growth decline immediately prior to the pulse. Maximum values of percent-growth-change decline exponentially as prior growth rate increases. This negative exponential rate is quantified as a boundary line, which is used to scale each percent-growth change pulse by the maximum possible value predicted by prior growth rate. The consistency of the relationship between radial growth prior to a release and the magnitude of the release is evaluated in multiple eastern hemlock data sets. Trees from diverse sites show large releases that approach the maximum value predicted by the prior-growth rates. These sites tend to have a history of disturbance, suggesting that disturbance is the most influential variable determining the magnitude of release response. Possible exceptions are sites on the northern and western borders of eastern hemlock's range, which consistently fall short of the boundary line and may be exhibiting unique relationships between prior growth and percent-growth change. Yet overall the relationship between prior growth and percent-growth change appears to be consistent across much of eastern hemlock's range. Furthermore, we demonstrate that similar boundary lines occur in Pinus ponderosa (Laws.), Pinus echinata (Mill.), Quercus alba (L.), and Quercus macrocarpa (Michx.). This suggests that prior growth could form the basis of a more unified set of release criteria better able to standardize release responses within and among species. Standardized release criteria would allow more accurate comparisons of disturbance histories among sites, species, and even across landscapes. 相似文献
Bayesian inference is an important statistical tool that is increasingly being used by ecologists. In a Bayesian analysis, information available before a study is conducted is summarized in a quantitative model or hypothesis: the prior probability distribution. Bayes’ Theorem uses the prior probability distribution and the likelihood of the data to generate a posterior probability distribution. Posterior probability distributions are an epistemological alternative to P‐values and provide a direct measure of the degree of belief that can be placed on models, hypotheses, or parameter estimates. Moreover, Bayesian information‐theoretic methods provide robust measures of the probability of alternative models, and multiple models can be averaged into a single model that reflects uncertainty in model construction and selection. These methods are demonstrated through a simple worked example. Ecologists are using Bayesian inference in studies that range from predicting single‐species population dynamics to understanding ecosystem processes. Not all ecologists, however, appreciate the philosophical underpinnings of Bayesian inference. In particular, Bayesians and frequentists differ in their definition of probability and in their treatment of model parameters as random variables or estimates of true values. These assumptions must be addressed explicitly before deciding whether or not to use Bayesian methods to analyse ecological data. 相似文献
A new method is described for the esterification of carboxyl groups in proteins by reaction of the lyophilized protein in vacuo with gaseous alcohol and HCI catalyst. Carboxyl groups are rapidly esterified with no protein degradation. 13C-Methyl or 13C-ethyl esters of the alpha-, gamma- and delta-carboxyl groups could be distinguished by the distinct chemical shifts of their resonances. Within the class of gamma- or delta-esters, the chemical shifts have little variation; however, the chemical shift of a C-terminal esterified alpha-carboxyl group shows a strong dependence on the nature of the C-terminal amino acid and sequence. Iodomethane reacts with deprotonated carboxyl groups in lyophilized proteins to form methyl esters, but unlike the reaction with gaseous methanol/HCI, it does not selectively methylate carboxyl groups. The procedure permits the cost-effective incorporation of isotopic labels and provides a new approach using 13C-NMR spectroscopy for determining the number of different C-termini present in a protein preparation. 相似文献