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Microwave Radiation: Biophysical Considerations and Standards Criteria

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Herman P. Schwan · 1972

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This 1972 study established thermal-only EMF safety standards that modern research suggests may be inadequate.

Plain English Summary

Summary written for general audiences

This 1972 foundational study by researcher Herman Schwan examined how microwave radiation interacts with human tissues and established early safety standards. Schwan distinguished between 'strong' field effects that require high power levels and 'weak' effects, concluding that non-thermal biological effects from microwaves were unlikely based on the electrical properties of cells and tissues.

Why This Matters

This paper represents a pivotal moment in EMF science - Herman Schwan's work became the foundation for virtually all modern microwave exposure standards, including those governing cell phones and WiFi today. Schwan's conclusion that non-thermal effects were 'unlikely' has been used for decades to justify safety standards based purely on heating effects. However, the science demonstrates this thermal-only approach may be inadequate. Since 1972, hundreds of peer-reviewed studies have documented biological effects from EMF exposures well below heating thresholds - effects Schwan deemed improbable. What this means for you: the safety standards protecting you from your smartphone, WiFi router, and other wireless devices are still largely based on 50-year-old assumptions that modern research increasingly challenges.

Finding

This statement applies for all reversible effects. An important part of the paper is concerned with standards of safe exposure. The difficulties to refine present standards are explained. Limit numbers of safe tissue-current density are proposed. They are useful as guides for tolerance standards, both in the presence of complex field configurations and at low frequencies.

In their words

“This statement applies for all reversible effects. An important part of the paper is concerned with standards of safe exposure. The difficulties to refine present standards are explained. Limit numbers of safe tissue-current density are proposed. They are useful as guides for tolerance standards, both in the presence of complex field configurations and at low frequencies.”

Figures from the Original Paper

Diagrams extracted from the original research document.

graphPage 2 - Fig. 1: Depth of penetration as a function of frequency for eye lens and vitreous humor.
p. 2 of the original PDF
graphPage 3 - Fig. 2: Relative absorption cross section of a sphere with tissue-like dielectric properties as function of sphere radius at three different frequencies.
p. 3 of the original PDF
diagramPage 4 - Figure 4. Classification of biological effects of nonionizing radiation (see text). Strong interactions occur at field-strength values higher than specified by the USASI exposure standard, and are therefore accompanied by a significant temperature rise (thermal effects).
p. 4 of the original PDF
graphPage 5 - AI-described figure: Figure 5 illustrates the time constant for pearl chain formation versus field strength, showing a relationship between E and T.
p. 5 of the original PDF
graphPage 7 - AI-described figure: Figure 6 illustrates threshold values for current densities as a function of frequency, highlighting excitation and sensation levels in relation to microwave frequencies.
p. 7 of the original PDF
chartPage 8 - AI-described figure: Figure 7 illustrates tolerance flux levels as a function of frequency for current densities of 1 and 3 mA/cm².
p. 8 of the original PDF

Exposure Information

Specific exposure levels were not quantified in this study.

Cite This Study
Herman P. Schwan (1972). Microwave Radiation: Biophysical Considerations and Standards Criteria.
Show BibTeX
@article{microwave_radiation_biophysical_considerations_and_standards_criteria_g5064,
  author = {Herman P. Schwan},
  title = {Microwave Radiation: Biophysical Considerations and Standards Criteria},
  year = {1972},
  
  
}
No DOI on file for this study.

Quick Questions About This Study

Schwan classified 'strong' effects as those requiring high field strengths, like pearl chain formation in cells. 'Weak' effects included potential membrane interactions, which he considered unlikely based on 1972 understanding of cellular electrical properties.
Based on 1972 knowledge of membrane and macromolecule electrical properties, Schwan concluded direct biological effects below heating thresholds were improbable. This thermal-only perspective became the foundation for current safety standards.
Modern FCC and international EMF exposure limits are still largely based on Schwan's thermal-only safety framework from 1972, despite decades of research documenting non-thermal biological effects he considered unlikely.
Schwan proposed specific current density limits as guides for exposure standards, suggesting these could be relaxed by orders of magnitude at frequencies below 30 MHz compared to microwave frequencies.
Schwan concluded pulsed microwave fields cannot cause biological effects any better than continuous fields of the same average power, though he noted macromolecular orientation by pulsed fields remained a remote possibility.