A determinação de situações de riscos na população que faz uso de agrotóxico é complexa. O objetivo desse estudo foi conhecer o contexto social, econômico e cultural e algumas das vulnerabilidades para a saúde relacionadas com o processo produtivo químico dependente da fruticultura do Vale do São Francisco. Questionário semiestruturado foi aplicado a uma amostra aleatória de trabalhadores rurais, para obtenção de informações socioeconômicas e culturais, assim como a morbidade relacionada com intoxicação por agrotóxicos. Os trabalhadores rurais da região têm em sua maioria baixo grau de escolaridade, sendo esta uma importante vulnerabilidade para compreensão da rotulagem dos agrotóxicos e sua implicação toxicológica e ambiental. Foi observado uso indiscriminado de agrotóxicos em condições inseguras de trabalho que comprometem a saúde dos expostos. A sintomatologia sugestiva de intoxicação por agrotóxicos aponta para quadros relacionados com exposição à organofosforados, carbamatos e piretróides, em congruência com os biocidas utilizados na fruticultura. 7% da população estudada referiram ter sofrido pelo menos um caso de intoxicação no decorrer da vida. A assistência técnica relacionada com o manejo de agrotóxicos é precária e não se observaram ações de proteção no âmbito da saúde, do trabalho, da previdência ou do ambiente. Estas vulnerabilidades institucionais corroboram aquelas relacionadas com o modelo tecnológico que objetiva meramente a produtividade e o rendimento financeiro.

We performed electric arc discharges in pure Si to generate luminous balls with lifetime in the order of seconds and several properties usually reported for natural ball lightning. This simple experiment does not rely on energy sources and excitation mechanisms that are improbable in the natural phenomenon and clearly demonstrates the role of vaporization and oxidation of Si, as proposed by the Abrahamson-Dinniss theory for ball-lightning formation.

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We report for the first time fully ab initio valence bond (VB) calculations with explicit use of the unsynchronized resonance structures introduced by Pauling [1]. We show that resonance involving these structures largely determines the stability and conformation of the Li 4 cluster and plays a central role in a VB explanation of the 3-center bonds in planar alkali clusters. The theory can make qualitative predictions on the behaviour of general metallic clusters, and can relate stability and conformation to electronic structure, thus indicating the origin of magic numbers. This first ab initio test of Pauling's resonating VB theory confirms the importance of the metallic orbital and the covalent character of the metal-metal bond. PACS: 31.10.+z; 31.20.Rx; 36.40+d; 71.10+x I IntroductionHistorically, two main approaches have been used to describe chemical bonds in molecules and solids: the molecular orbital (MO) and valence bond (VB) theories. For metals, however, only the band theory (the crystal analogue of the MO approach) has been fully accepted and widely used in discussing electronic structure and properties. The reason seems to be not only the technical difficulty of making VB calculations but also the long-standing belief, going back to Bragg [2], that localized covalent bonds cannot be present in metals. Only Pauling, many years ago, put forward an alternative VB theory [1], albeit based on mainly empirical arguments. More recently, statistical arguments have been advanced in support of these ideas [3] and have been used in the interpretation of the superconductivity of some copper oxide compounds [4].In this paper we first sketch Pauling's approach, also known as resonating valence bond (RVB) theory, and briefly summarize some of the conclusions reached using the more orthodox MO methods. In following sections we outline our (non-empirical) RVB calculations, making comments on various aspects of the VB approach and its implementation, introduce an RVB explanation of some properties of metal clusters, and finally discuss the implications of our results.The main premises of Pauling's approach are as follows: 1. In metals the atoms possess an orbital free to receive electrons, the metallic orbital, in addition to those occupied by bonding electrons and unshared electron pairs. 2. In addition to the synchronized resonance among the usual covalent Kekulé type structures (which always show bonds between alternative pairs of atoms and in which displacement of one link implies the simultaneous displacement of another) there can be unsynchronized resonance in which the same atom, by invoking the metallic orbital, can acquire two electronpair bonds i.e. gain a second link without losing the first: the new structures will be referred to as Pauling structures. 3. By the principle of electroneutrality net charges higher than ±1 will not be found on any atom.Pauling used empirical and pictorial arguments to suggest that unsynchronized resonance would not only confer increased stability on a metallic lattice but would...

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