Heating of tissues by microwaves: a model analysis.

Bioelectromagnetics · 1998 · Unclear · Insufficient evidence

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UnclearInsufficient evidencePaper 6344

Study snapshot

PublicationBioelectromagnetics · 1998

Study typeengineering

Reported directionUnclear

Evidence ratingInsufficient

Abstract

We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau1) pertains to heat convection by blood flow, and is of the order of 20-30 min for physiologically normal perfusion rates; the second (tau2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau1 and tau2. In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard.

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AI evidence extraction

Structured fields below were generated from the paper’s abstract and metadata to make the evidence easier to inspect. Automated extraction may be incomplete or incorrect; verify important details against the original paper.

Study typeengineering

Effect directionunclear

PopulationNot supplied

Sample sizeNot supplied

Exposuremicrowave

Evidence strengthInsufficientConfidence: 74% · Peer reviewed: yes

Main findings

Using a dimensionless bioheat-equation analysis, the thermal response to microwave/infrared irradiation is governed by two time constants: a perfusion-related convection constant (tau1, ~20–30 min for normal perfusion) and a conduction constant (tau2) that scales with the square of a characteristic heating distance. For two idealized geometries (surface-area exposure with exponentially decreasing SAR; localized hemispherical uniform SAR), the steady-state temperature rise can be expressed as incident power density times an effective time constant tau(eff) derived from tau1 and tau2; in limiting cases, temperature rise is dominated by conductive or convective transport. The block-model predictions are reported to agree well with recent data on thresholds for warmth/pain perception from microwave exposure, and the authors propose a thermal averaging time concept for standards and compare ANSI microwave and infrared laser standards in terms of allowed maximal tissue temperature increase.

Outcomes measured

Tissue temperature increase (heating)
Thermal response time constants (convection/perfusion and conduction)
Thermal averaging time for exposure standards
Thresholds for perception of warmth or pain (comparison to data)
Comparison of ANSI exposure standards (microwaves vs infrared laser) for allowed maximal tissue temperature increase

Limitations

Model-based analysis with idealized geometries (tissue block with insulated surface; hemispherical region)
No specific exposure frequencies, SAR values, or power densities reported in the abstract
No direct experimental study details (population/sample size) provided in the abstract

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": "microwave",
    "source": null,
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": null,
  "sample_size": null,
  "outcomes": [
    "Tissue temperature increase (heating)",
    "Thermal response time constants (convection/perfusion and conduction)",
    "Thermal averaging time for exposure standards",
    "Thresholds for perception of warmth or pain (comparison to data)",
    "Comparison of ANSI exposure standards (microwaves vs infrared laser) for allowed maximal tissue temperature increase"
  ],
  "main_findings": "Using a dimensionless bioheat-equation analysis, the thermal response to microwave/infrared irradiation is governed by two time constants: a perfusion-related convection constant (tau1, ~20–30 min for normal perfusion) and a conduction constant (tau2) that scales with the square of a characteristic heating distance. For two idealized geometries (surface-area exposure with exponentially decreasing SAR; localized hemispherical uniform SAR), the steady-state temperature rise can be expressed as incident power density times an effective time constant tau(eff) derived from tau1 and tau2; in limiting cases, temperature rise is dominated by conductive or convective transport. The block-model predictions are reported to agree well with recent data on thresholds for warmth/pain perception from microwave exposure, and the authors propose a thermal averaging time concept for standards and compare ANSI microwave and infrared laser standards in terms of allowed maximal tissue temperature increase.",
  "effect_direction": "unclear",
  "limitations": [
    "Model-based analysis with idealized geometries (tissue block with insulated surface; hemispherical region)",
    "No specific exposure frequencies, SAR values, or power densities reported in the abstract",
    "No direct experimental study details (population/sample size) provided in the abstract"
  ],
  "evidence_strength": "insufficient",
  "confidence": 0.74,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "microwaves",
    "infrared radiation",
    "nonionizing radiation",
    "bioheat equation",
    "specific absorption rate",
    "tissue heating",
    "thermal response time",
    "perfusion",
    "heat conduction",
    "thermal averaging time",
    "ANSI exposure standards",
    "pulsed sources"
  ],
  "suggested_hubs": [
    {
      "slug": "who-icnirp",
      "weight": 0.62,
      "reason": "Discusses concepts relevant to human exposure standards (thermal averaging time) and compares ANSI standards for microwave/infrared exposure."
    }
  ]
}

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AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.

Identifiers and source

DOI: 10.1002/(sici)1521-186x(1998)19:7<420::aid-bem3>3.0.co;2-3
PubMed: 9771585

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