We strongly agree with the authors that, although there is no compelling evidence to suggest that nonionizing electromagnetic fields represent a public health hazard, there is sufficient evidence of magnetic- and electric field-induced biologic effects to continue scientific investigation of this issue.
We strongly agree with the authors that, although there is nocompelling evidence to suggest that nonionizing electromagneticfields represent a public health hazard, there is sufficient evidenceof magnetic- and electric field-induced biologic effects to continuescientific investigation of this issue.
Just as it would be inadequate to investigate the action of afew chemicals and decide that there is no harm possible from anychemical in the environment, a more informed strategy is neededin studies of electric and magnetic fields. Salvatore et al rightlysuggest that nonionizing electromagnetic fields are characterizedby a number of parameters. In fact, because fields are uniquelycharacterized only when the value of each parameter (frequency,magnitude, orientation, duration, and so on) is defined, it isgenerally not realistic to draw direct analogies between fieldexposure and single chemical exposures. For example, "highexposure" to fields refers only to the measured field magnitude,and not necessarily to its biologic effectiveness.
Although epidemiologic studies have, in some cases, shown a correlationbetween nonionizing electromagnetic field exposure and the developmentof certain cancers, the studies have not yet clearly identifiedspecific details of nonionizing electromagnetic fields that mightbe causative. Laboratory studies, conducted under well-definedconditions, may be useful for determining whether the exposurecategories used in epidemiologic studies are adequate, or howthey can be improved.
Essentially, all studies of nonionizing electromagnetic fieldsfocus on magnetic field effects because magnetic fields are rarelyshielded (attenuated) in the inhabited environment (home, work,or transportation). Furthermore, laboratory studies have shownthat magnetic fields can have direct action on biologic processes(eg, Blackman et al ), and epidemiologic data appear to bemore strongly correlated with estimates of magnetic fields thanwith electric field estimates .
The Need to Define "Dose"
Until now, most laboratory animal studies using particular cancermodels with various nonionizing electromagnetic field exposuresshowed negative results. However, at least two models (a skintumor model [3,4], and a breast tumor model [5-7]have recentlyshown positive effects under specific, precise nonionizing electromagneticfield treatment conditions. These studies suggest the need forrefining what is meant by "dose" under nonionizing electromagneticfield exposures.
Three particularly promising areas of laboratory research forproviding critical focus to research are:
1. Investigations of subtle distinctions in field exposure conditionsthat are necessary to elicit biologic changes,
2. Determination of whether and how certain magnetic field exposuresmay augment the action of chemicals known to influence cancerdevelopment, and
3. Studies examining how certain magnetic field exposures mayinduce changes in the production, availability, and action ofcritical oncostatic hormones, such as melatonin.
Characterizing Field Exposure Conditions
Recent laboratory research suggests that nonionizing electromagneticfield interactions with biologic systems may be resonance-basedto some extent. Several resonance interaction models, with differentdegrees of predictivity and validation, exist. For example, theion cyclotron resonance (ICR) model  identifies combinationsof alternating-current (AC) frequency and direct-current (DC)magnetic flux density required to establish resonance conditionsfor biologically active ions. Lednev  augmented the ICR modelby including the predicted influence of the AC magnetic fieldflux density (Bac) on the resonance process.
More recently, the ion parametric resonance (IPR) model  identifieda Bac-based response form different from the Lednev model, andconsiders the combined influence of multiple ion resonances atany given exposure condition. This IPR model has growing experimentalsupport [11,12]. Other well-established magnetic resonance models,such as nuclear magnetic resonance and electron spin resonance,also appear to be useful in identifying the precise conditionsof nonionizing electromagnetic field exposure to create biologicchanges . In each model, fundamental magnetic field conditionsmust be closely controlled, including AC frequency(ies), AC andDC flux densities, and the relative orientation between the ACand DC magnetic field vectors.
Nonionizing electromagnetic fields may also interact through inducedelectric current in conductive biologic materials. It has beensuggested, for example, that externally generated electric transientsmay lead to more biologically significant induced currents withinthe body.
Other studies examined the time signature of exposure conditions.Historically, epidemiologic studies assumed that time-weightedaverage (TWA) magnetic flux density was a sufficient measure ofexposure. The most compelling of these studies  showed thataveraging exposure intensity over 1-year periods produced a highercorrelation with disease incidence than spot measurements takenfor only a 15-minute-period. However, several recent studies (forexample, Morgan and Nair ), challenge the use of the TWA metric,and have searched for more appropriate measures.
Synergy With Chemicals
The biologic effects of nonionizing electromagnetic field exposuresmay be influenced by the presence of certain chemicals withinthe body. These include environmental chemicals, specifically,tumor promoters, and indigenous signaling molecules, such as hormones.Tumor promoters are believed to influence biologic systems onlywhen their concentrations are above a certain threshold value.As noted by Salvatore et al, it is possible that magnetic fieldscan change (reduce) that apparent threshold.
Hormones normally carry information between tissue systems toprovide timely stimuli, causing specific tissue/cell metabolicresponses. For example, the pineal hormone, melatonin, dramaticallyinfluences the body's circadian clock. Melatonin is the most potentfree-radical scavenger in the body (with potential implicationsfor protection against cancer-initiating events)  and alsoacts as a modulator of gap junction intercellular communication, an information channel that both coordinates cellular functionsand allows neighboring cells to control the growth of "initiated"cells. Recent studies suggest that nonionizing electromagneticfield exposures may control the amount of circulating melatoninand influence its specific functions, making the biologic systemmore susceptible to disease.
Because nonionizing electromagnetic fields cannot be characterizedsolely by their magnitude, they are more complicated than earlierstudies have assumed, and hence, the single chemical analogy isinappropriate. Present research suggests that (1) specific combinationsof nonionizing electromagnetic fields may be more effective atcreating biologic effects than others, and (2) synergistic effectsbetween chemicals and nonionizing electromagnetic fields are possiblewithin a biologic system. It is too early to claim categoricallythat nonionizing electromagnetic fields do or do not have carcinogenicproperties. More directed research is clearly required to elucidatethe biologic significance of ubiquitous nonionizing electromagneticfields exposure(s).
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