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Microwave Radiation: Biophysical Considerations and Standards Criteria

Bioeffects Seen

Herman P. Schwan · 1972

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This foundational 1972 study established thermal-only EMF safety standards that may not account for modern pulsed wireless signals.

Plain English Summary

Summary written for general audiences

This 1972 foundational study by biophysicist H.P. Schwan examined how microwave radiation interacts with human tissues and established early safety standards. Schwan classified biological effects as either 'strong' (requiring high field strengths) or 'weak,' and argued that pulsed microwave fields cannot cause more biological damage than continuous fields of the same average power.

Why This Matters

This study represents a pivotal moment in EMF research history - one of the earliest systematic attempts to understand microwave-biological interactions and establish safety criteria. Schwan's work laid the theoretical foundation for thermal-only safety standards that still govern EMF regulation today. His conclusion that pulsed fields cannot exceed continuous field effects in biological damage has been challenged by decades of subsequent research showing unique effects from pulsed and modulated signals. What makes this particularly relevant is that modern wireless devices - from WiFi routers to 5G networks - predominantly use pulsed and modulated signals that weren't fully considered in these early frameworks. The science demonstrates that Schwan's 1972 assumptions about equivalent biological effects may not account for the complex signaling patterns your devices use today.

Finding

This statement applies for all of the above considerations. An important part of the paper is concerned with standards of safe exposure. The difficulties to refine present standards are explained. Plans for a safer 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 of the above considerations. An important part of the paper is concerned with standards of safe exposure. The difficulties to refine present standards are explained. Plans for a safer 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 - Figure 1. Depth of penetration as 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 - AI-described figure: Fig. 4: Classification of biological effects of nonionizing radiation showing strong and weak interactions in relation to U.S.A. Standard Institute standard.
p. 4 of the original PDF
graphPage 5 - AI-described figure: Fig. 5 illustrates the relationship between time constant and field strength for pearl chain formation versus field strength.
p. 5 of the original PDF
chartPage 7 - AI-described figure: Figure 6 illustrates threshold values for current densities as a function of frequency, with shaded areas indicating 'excitation' and 'sensation' levels based on biophysical considerations.
p. 7 of the original PDF
chartPage 8 - 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_g4023,
  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 defined 'strong' effects as those requiring high field strengths, like pearl chain formation in cell organelles. 'Weak' effects included potential membrane and molecular interactions that he considered unlikely at typical exposure levels.
Schwan argued that pulsed microwave fields cannot cause more biological effects than continuous fields of the same average power, dismissing concerns about unique pulsed signal interactions with biological systems.
Schwan's thermal-based framework became the foundation for current EMF safety standards, focusing primarily on tissue heating effects while largely dismissing non-thermal biological interactions at lower power levels.
Schwan noted significant limitations in animal experimentation for understanding human microwave effects, advocating instead for biophysical principles and theoretical modeling to establish safety criteria and exposure limits.
Schwan acknowledged potential non-thermal effects on biological membranes and macromolecules but dismissed them as unlikely and unproven, arguing insufficient knowledge existed about electrical properties of these biological materials.