Microwave hazards

Dawkins, A. W. J., N. R. V. Nightingale, G. P. South, R. J. Sheppard, and E. H. Grant, 1979. The role of water in microwave absorption by biological material with particular reference to microwave hazards, Phys. Med. Biol., 24, 1168-1176. 

Peters and Miethschen, and the hazards of HF generation, 524 Phase detection experiments, with microwaves, 451... 

The authors have also elaborated a microwave-enhanced one-pot procedure [90] for the Huisgen 1,3-dipolar cycloaddition reaction. In a typical procedure, a pyrazinone with a triple bond connected to the core via C - O linkage, was reacted with a suitable benzylic bromide and NaNs in presence of the Cu(I) catalyst in a t Bu0H/H20 system under microwave irradiation (Scheme 26). The cycloaddition was found to proceed cleanly and with full regioselectivity. As the azide is generated in situ, this procedure avoids the isolation and purification of hazardous azides, which is especially important when handling the ahphatic ones, which are known to be toxic and explosive in nature. 

Cao, S.W., Zhu, Y.J., Cheng, G.F. and Huang, Y.H. (2009) ZnFe204 nanopartides microwave—hydrothermal ionic liquid synthesis and photocatalytic property over phenol. Journal of Hazardous materials, 171 (1—3), 431—435. 

Laboratory Safety From a laboratory safety standpoint, both open and closed systems have mechanisms for the disposal of hazardous wastes, thus reducing the technician s exposure to potentially toxic materials. Performing HIAR online (closed system) as opposed to off-line mechanisms (microwave ovens or steamers) removes the possibility of being burned while handling hot containers or boiling liquids. 

Precautions should be taken, especially in a scale-up approach, when dealing with exothermic reactions in the microwave field. Due to the rapid energy transfer of microwaves, any uncontrolled exothermic reaction is potentially hazardous (thermal runaway). Temperature increase and pressure rise may occur too rapidly for the instrument s safety measures and cause vessel rupture. 

Operating with chemicals and pressurized containers always carries a certain risk, but the safety features and the precise reaction control of the commercially available microwave reactors protect the users from accidents, perhaps more so than with any classical heating source. The use of domestic microwave ovens in conjunction with flammable organic solvents is hazardous and must be strictly avoided as these instruments are not designed to withstand the resulting conditions when performing chemical transformations. 

Economic and safety considerations encourage minimal stockpiling of chemicals and avoiding transportation of hazardous substances. These increasing demands offer many opportunities for microwave chemistry in the development of environmentally benign methods for the preparation of intermediates, specialty chemicals and pharmaceuticals [6[. It appears likely that within the next few years, individual chemical reactors will be required for diverse tasks and will need to be easily relocated... 

To avoid explosion hazards [70], a modified domestic microwave oven was used. Higher yields were obtained in shorter periods of time on use of microwave irradiation (39% yield after 20 min irradiation) in comparison with thermal conditions. Longer irradiation times led to a decrease in the yield of 75, because of increased bis adduct formation. 

The use of microwave techniques introduces unique challenges in safety considerations. Guidelines for the use of microwave systems in the analytical laboratory have been published and most of these also apply to microwave-assisted organic chemistry [13]. The health hazards of microwave radiation are also still under investigation, and it is not yet known whether or not low level exposure is detrimental. Recom-... 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

The possibility of microwaves being a hazard to health has been investigated extensively, and much care has been spent on optimizing safety features, so it is now clear that microwave ovens are completely safe. 

Power tubes, microwave, 26 520 Power washing, of metal surfaces, 26 213 Poxviruses, 3 136 Poynting correction, 8 746 24 666 Pozzolanic materials, in hazardous waste management, 25 823 Pozzolans, 5 478—479, 493 U.S. shipments, 5 498t PPA resins, properties of, 10 216t PPE-PS blends, 20 193 PPG-10 cetyl ether... 

Method 3052. Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices, http //www.epa.gov/epawaste/hazard/testmethods/sw846. 2012. Available at http //www.epa.gov/epawaste/hazard/testmethods/sw846. Accessed June 3, 2013. 

The pesticide industry generates many concentrated wastes that are considered hazardous wastes. These wastes must be detoxified, pretreated, or disposed of safely in approved facilities. Incineration is a common waste destruction method. Deep well injection is a common disposal method. Other technologies such as wet air oxidation, solvent extraction, molten-salt combustion, and microwave plasma destmction have been investigated for pesticide waste applications. 

The Emery microwave process is a patented, ex situ technology for the treatment of soil and water contaminated with polychlorinated biphenyls (PCBs) and other hazardous compounds. The vendor states that the technology uses microwave energy to break the bonds of long-chain hydrocarbons. 

Regardless of the truth of these stories, it is undeniable that microwave radiation can be hazardous to life. One damage mechanism is merely intense heating of the water bound up in all living organisms. It is clear, therefore, that the potential hazard of radiation of a given frequency depends on the optical constants, particularly c", of water at that frequency. For example, at room temperature the maximum value of c" occurs at about 20 GHz (see Fig. 9.15), that is, at the relaxation frequency 1/2 ttt, where r is the relaxation time in (9.41). 

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