William F. Christensen

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During the past three decades, receptor models have been used to identify and apportion ambient concentrations to sources. A number of groups are employing these methods to provide input into air quality management planning. A workshop has explored the use of resolved source contributions in health effects models. Multiple groups have analyzed particulate(More)
Source apportionment may be useful in epidemiological investigation of PM health effects, but variations and options in these methods leave uncertainties. An EPA-sponsored workshop investigated source apportionment and health effects analyses by examining the associations between daily mortality and the investigators' estimated source-apportioned PM(2.5)(More)
A systematic approach was used to quantify the metals present in particulate matter emissions associated with on-road motor vehicles. Consistent sampling and chemical analysis techniques were used to determine the chemical composition of particulate matter less than 10 microm in aerodynamic diameter (PM10*) and particulate matter less than 2.5 microm in(More)
Although the association between exposure to ambient fine particulate matter with aerodynamic diameter < 2.5 microm (PM2.5) and human mortality is well established, the most responsible particle types/sources are not yet certain. In May 2003, the U.S. Environmental Protection Agency's Particulate Matter Centers Program sponsored the Workshop on the Source(More)
OBJECTIVES The aims of this study were to determine the feasibility of obtaining auditory cortex activation evoked by pure tones presented at threshold and suprathreshold hearing levels, to evaluate tonotopicity of the primary auditory cortex, and to determine the effect of stimulus intensity on auditory cortex activation using silent functional magnetic(More)
Although much of the media attention during presidential election years focuses on polls tracking popular support for the major candidates, the complicated role played by the Electoral College in this multistage election process must be accounted for in order to address the issue of winning the presidency. State-level pre-election polls are used in a manner(More)
Functional magnetic resonance imaging (fMRI) allows neuroscientists to assess brain function by evaluating haemodynamic activity (blood flow) when a stimulus is present or absent. In clinical practice, the hearing levels of individuals are determined using an audiometer that allows presentation of a pure-tone of specific intensity and frequency. Functional(More)
PURPOSE To evaluate auditory cortex activation evoked by pure-tone stimulus using silent functional magnetic resonance imaging (FMRI). MATERIAL AND METHODS Nine volunteers with normal hearing as determined with pure-tone audiometry were studied. Auditory cortex activation was evoked by pure-tone stimuli presented monaurally at 1,000, 2,000, and 4,000 Hz.(More)
OBJECTIVE To evaluate cardiac function parameters in a group of active and hibernating grizzly bears. DESIGN Prospective study. ANIMALS 6 subadult grizzly bears. PROCEDURE Indirect blood pressure, a 12-lead ECG, and a routine echocardiogram were obtained in each bear during the summer active phase and during hibernation. RESULTS All measurements of(More)