Abstract:
mercury (hg) is currently considered a global pollutant. mercury is a signifcant environmental toxin and has a huge impact on human health, causing irreversible damage to the central nervous system, especially in the most vulnerable stages of life. medical centers contribute to 4-5% of the total mercury present in drainage waters. practices should be introduced to reduce mercury use with the aim of complete elimination. international entities, governments, non-governmental organizations, and health professionals should support all measures taken to achieve this goal.

Abstract:
A common feature of reparametrization invariant theories is the difficulty involved in identifying an appropriate evolution parameter and in constructing a Hilbert space on states. Two well known examples of such theories are the relativistic point particle and the canonical formulation of quantum gravity. The strong analogy between them (specially for minisuperspace models) is considered in order to stress the correspondence between the ``localization problem'' and the ``problem of time,'' respectively. A possible solution for the first problem was given by the proper time formulation of relativistic quantum mechanics. Thus, we extrapolate the main outlines of such a formalism to the quantum gravity framework. As a consequence, a proposal to solve the problem of time arises.

Abstract:
We consider a particular (exactly soluble) model of the one discussed in a previous work. We show numerical results for the time evolution of the main dynamical quantities and compare them with analytical results.

Abstract:
A great effort has been devoted to formulate a classical relativistic theory of spin compatible with quantum relativistic wave equations. The main difficulty in order to connect classical and quantum theories rests in finding a parameter which plays the role of proper time at a purely quantum level. We present a partial review of several proposals of classical and quantum spin theories from the pioneer works of Thomas and Frenkel, revisited in the classical BMT work, to the semiclassical model of Barut and Zanghi [Phys. Rev. Lett. 52, 2009 (1984)]. We show that the last model can be obtained from a semiclassical limit of the Feynman proper time parametrization of the Dirac equation. At the quantum level we derive spin precession equations in the Heisenberg picture. Analogies and differences with respect to classical theories are discussed in detail.

Abstract:
A new synthesis of the principles of relativity and quantum mechanics is developed by replacing the Poincar\'e group for the de Sitter one. The new relativistic quantum mechanics is an indefinite mass theory which is reduced to the standard theory on the mass shell. The charge conjugation acquires a geometrical meaning and the Stueckelberg interpretation for antiparticles naturally arises in the formalism. So the idea of the Dirac sea in the second quantized formalism proves to be superfluous. The off-shell theory is free from ultraviolet divergences, which only appear in the process of mass shell reduction.

Abstract:
The genesis of Feynman's original approach to QED is reviewed. The main ideas of his original presentation at the Pocono Conference are discussed and compared with the ones involved in his action-at-distance formulation of classical electrodynamics. The role of the de Sitter group in Feynman's visualization of space-time processes is pointed out.

Abstract:
The hyperplane and proper time formalisms are discussed mainly for the spin-half particles in the quantum case. A connection between these covariant Hamiltonian formalisms is established. It is showed that choosing the space-like hyperplanes instantaneously orthogonal to the direction of motion of the particle the proper time formalism is retrieved on the mass shell. As a consequence, the relation between the St\"uckelberg-Feynman picture and the standard canonical picture of quantum field theory is clarified.

Abstract:
A many particle Hamiltonian, where the interaction term conserves the number of particles, is considered. A master equation for the populations of the different levels is derived in an exact way. It results in a local equation with time-dependent coefficients, which can be identified with the transition probabilities in the golden rule approximation. A reinterpretation of the model as a set of coupled harmonic oscillators enables one to obtain for one of them an exact local Langevin equation, with time-dependent coefficients.

Abstract:
This work is a comment on Ryder's derivation of the Dirac equation, with emphasis on the physical contents of this equation: the notion of particles and antiparticles according to the Stueckelberg-Feynman interpretation, the opposite intrinsic parity between particles and antiparticles, and the spin.

Abstract:
arsenic (as) is a semimetal that is widely distributed in nature, in water and soil. in argentine, the contamination of both waterways and groundwater represents the main environmental problem caused by this element. chronic as poisoning is known as chronic endemic regional hydroarsenicism (c.e.r.ha.). long-term exposure to low concentrations of the element from the prenatal period onward results in the well-known symptoms of chronic as poisoning. cerha develops progressively, compromising different organs and systems, most importantly the skin. one of the most important complications of cerha is de development of neoplasias, mainly skin tumors. childhood environmental health is a challenge in the new millennium and health care professionals play a fundamental role in the protection against environmental hazards such as chronic arsenic poisoning.