Predicting attenuant and resonant 2-cycles in periodically forced discrete-time two-species population models.

Periodic environments may either enhance or suppress a population via resonant or attenuant cycles. We derive signature functions for predicting the responses of two competing populations to 2-periodic oscillations in six model parameters. Two of these parameters provide a non-trivial equilibrium and two provide the carrying capacities of each species in the absence of the other, but the remaining two are arbitrary and could be intrinsic growth rates. Each signature function is the sign of a weighted sum of the relative strengths of the oscillations of the perturbed parameters. Periodic environments are favourable for populations when the signature function is positive and are deleterious if the signature function is negative. We compute the signature functions of four classical, discrete-time two-species populations and determine regions in parameter space which are either favourable or detrimental to the populations. The six-parameter models include the Logistic, Ricker, Beverton-Holt, and Hassell models.

An assessment of the occupational hazards related to medical lasers.

OBJECTIVE: Occupational hazards associated with medical laser applications remain poorly understood and uncharacterized. METHODS: A literature search was performed using PubMed, and all articles relevant to beam and nonbeam medical laser hazards were reviewed. The Rockwell Laser Industries Laser Accident Database was searched for medical laser injuries and abstracted. RESULTS: Eye injuries, skin burns, injuries related to the onset of fires, and electric shock have been reported in relation to medical laser use. It is probable that both acute and chronic health effects have been experienced by medical personnel as the result of exposure to laser generated air contaminants. CONCLUSIONS: Because of the clinical benefits they provide, the growth of laser technologies and applications are anticipated to result in an increase in the number and type of medical personnel with future exposure to laser hazards.

Laser-generated air contaminants from medical laser applications: a state-of-the-science review of exposure characterization, health effects, and control.

The clinical use of lasers in surgery began in 1973 with applications of the carbon dioxide laser in otolaryngology, and since then the use of lasers has become commonplace in many medical and surgical specialties. Nonetheless, when biological tissue is subjected to laser radiation, the target cells can be vaporized, resulting in the aerosolization of their contents and the subsequent exposure of health care workers to laser-generated air contaminants (LGACs). The purpose of our analysis was to summarize and present all of the published literature pertaining to the laser-induced plume chemical and physical composition, health effects, and methods of control. The objective was to identify knowledge gaps within exposure science to set a research agenda for the protection of health care personnel exposed to LGACs. A literature search was performed using the PubMed database using a variety of search strategies and keyword combinations. To locate additional studies, we systematically searched the reference lists of all studies identified by our search, as well as key review papers. To date, researchers have identified roughly 150 chemical constituents of plume, as well as fine and ultrafine particulate matter, which has been shown to include viable cellular material, viruses, and bacteria. However, very few studies have attempted to characterize the effects of laser system type, power, and tissue treated, as it relates to LGAC exposure. Furthermore, current control strategies do not appear to be adequate in preventing occupational exposure to LGACs.

Periodically forced discrete-time SIS epidemic model with disease induced mortality.

We use a periodically forced SIS epidemic model with disease induced mortality to study the combined effects of seasonal trends and death on the extinction and persistence of discretely reproducing populations. We introduce the epidemic threshold parameter, R0 , for predicting disease dynamics in periodic environments. Typically, R0 <1 implies disease extinction. However, in the presence of disease induced mortality, we extend the results of Franke and Yakubu to periodic environments and show that a small number of infectives can drive an otherwise persistent population with R0 >1 to extinction. Furthermore, we obtain conditions for the persistence of the total population. In addition, we use the Beverton-Holt recruitment function to show that the infective population exhibits period-doubling bifurcations route to chaos where the disease-free susceptible population lives on a 2-cycle (non-chaotic) attractor.

Disease-induced mortality in density-dependent discrete-time S-I-S epidemic models.

The dynamics of simple discrete-time epidemic models without disease-induced mortality are typically characterized by global transcritical bifurcation. We prove that in corresponding models with disease-induced mortality a tiny number of infectious individuals can drive an otherwise persistent population to extinction. Our model with disease-induced mortality supports multiple attractors. In addition, we use a Ricker recruitment function in an SIS model and obtained a three component discrete Hopf (Neimark-Sacker) cycle attractor coexisting with a fixed point attractor. The basin boundaries of the coexisting attractors are fractal in nature, and the example exhibits sensitive dependence of the long-term disease dynamics on initial conditions. Furthermore, we show that in contrast to corresponding models without disease-induced mortality, the disease-free state dynamics do not drive the disease dynamics.

SIS epidemic attractors in periodic environments.

The demographic dynamics are known to drive the disease dynamics in constant environments. In periodic environments, we prove that the demographic dynamics do not always drive the disease dynamics. We exhibit a chaotic attractor in an SIS epidemic model, where the demograhic dynamics are asymptotically cyclic. Periodically forced SIS epidemic models are known to exhibit multiple attractors. We prove that the basins of attraction of these coexisting attractors have infinitely many components.

Globally attracting attenuant versus resonant cycles in periodic compensatory Leslie models.

We use a periodically forced density-dependent compensatory Leslie model to study the combined effects of environmental fluctuations and age-structure on pioneer populations. In constant environments, the models have globally attracting positive fixed points. However, with the advent of periodic forcing, the models have globally attracting cycles. We derive conditions under which the cycle is attenuant, resonant, and neither attenuant nor resonant. These results show that the response of age-structured populations to environmental fluctuations is a complex function of the compensatory mechanisms at different life-history stages, the fertile age classes and the period of the environment.

Personal dust exposures at a food processing facility.

A field study was performed to quantify personal dust exposures at a food processing facility. A review of the literature shows very little exposure information in the food processing industry. The processing area consisted of a series of four rooms, connected by a closed-loop ventilation system, housed within a larger warehouse-type facility. Workers were exposed to various fruit and vegetable dusts during the grinding, sieving, mixing and packaging of freeze-dried or air-dried products. Eight two-hour periods were monitored over two days. Personal total suspended particulate samples were collected on 37 mm PVC filters with 5 microm pore size according to National Institute for Occupational Safety and Health (NIOSH) Method 0500. The filters were analyzed gravimetrically. The two-hour task sampling personal dust exposures ranged from 0.33-103 mg/m3. For each worker, an eight-hour time weighted average (TWA) concentration was calculated, and these ranged from 3.08-59.8 mg/m3. Although there are no directly appropriate occupational exposure limits that may be used for comparison, we selected the Threshold Limit Value (TLV) for particulates not otherwise classified (PNOC) of 10 mg/m3 for inhalable particles. Neglecting the respiratory protection used, five out of eight of the worker time-weighted averages exceeded the TLV. It should be noted that the TLV is based on the inhalable fraction and in this study total suspended particulate was measured; additionally, the TLV is applicable for dusts that are insoluble or poorly soluble, and have low toxicity, which may have limited protective ability in this case due to the irritant nature of certain dusts (e.g., jalapeno peppers, aloe vera). Sieving resulted in significantly higher exposure than grinding and blending. Measuring area concentrations alone in this environment is not a sufficient method of estimating personal exposures due to work practices for some operations.

Signature function for predicting resonant and attenuant population 2-cycles.

Populations are either enhanced via resonant cycles or suppressed via attenuant cycles by periodic environments. We develop a signature function for predicting the response of discretely reproducing populations to 2-periodic fluctuations of both a characteristic of the environment (carrying capacity), and a characteristic of the population (inherent growth rate). Our signature function is the sign of a weighted sum of the relative strengths of the oscillations of the carrying capacity and the demographic characteristic. Periodic environments are deleterious for populations when the signature function is negative. However, positive signature functions signal favorable environments. We compute the signature functions of six classical discrete-time single species population models, and use the functions to determine regions in parameter space that are either favorable or detrimental to the populations. The two-parameter classical models include the Ricker, Beverton-Holt, Logistic, and Maynard Smith models.