Discussion on Spatial and Time Averaging Restrictions Within the Electromagnetic Exposure Safety Framework

2019 · Harm · Low evidence

← Research stream

HarmLow evidencePaper 1623

Study snapshot

Publicationmanual · 2019

Study typeengineering

Reported directionHarm

Evidence ratingLow

Abstract

Discussion on Spatial and Time Averaging Restrictions Within the Electromagnetic Exposure Safety Framework in the Frequency Range Above 6 GHz for Pulsed and Localized Exposures Neufeld E, Samaras T, Kuster N. Discussion on Spatial and Time Averaging Restrictions Within the Electromagnetic Exposure Safety Framework in the Frequency Range Above 6 GHz for Pulsed and Localized Exposures. Bioelectromagnetics. 2019 Dec 30. doi: 10.1002/bem.22244. Abstract Both the current and newly proposed safety guidelines for local human exposure to millimeter-wave frequencies aim at restricting the maximum local temperature increase in the skin to prevent tissue damage. In this study, we show that the application of the current and proposed limits for pulsed fields can lead to a temperature increase of 10°C for short pulses and frequencies between 6 and 30 GHz. We also show that the proposed averaging area of 4 cm2 , that is greatly reduced compared with the current limits, does not prevent high- temperature increases in the case of narrow beams. A realistic Gaussian beam profile with a 1 mm radius can result in a temperature increase about 10 times higher than the 0.4°C increase the same averaged power density would produce for a plane wave. In the case of pulsed narrow beams, the values for the time and spatial-averaged power density allowed by the proposed new guidelines could result in extreme temperature increases. ncbi.nlm.nih.gov Excerpts .... In this letter, we look at limits, such as those currently proposed or recently approved for the revised ICNIRP guidelines and IEEE standard, and investigate whether such limits are consistent with the stated goals of the exposure safety frameworks of preventing excessive heating in the case of pulsed and/or localized radiation. In cases when they are not consistent, we discuss how consistency can be achieved. In line with the abovementioned safety standards and exposure guidelines, the presented analysis focuses exclusively on the magnitude of the tissue temperature increase as a risk factor and does not consider other aspects, such as the thermoelastic effect related to the rapidity of temperature increase..... In conclusion, the results presented above demonstrate that, in the case of very short pulses, pulse-duration- independent limits imposed on transmitted energy density (fluence) alone cannot preclude the induction of high-temperature increases in the skin. Pulse-duration-dependent limits should be applied also for pulses less than 1 s and possibly less than 30 GHz as well. Even though the amplifiers of the currently developed consumer devices will not allow the full exploitation of the limits of the guidelines, the guidelines should not implicitly rely on this, as they will be used to develop exposure assessment standards with the aim of ensuring safety of any future technology, e.g. IEC/IEEE 63195 [2018]. Accordingly, either assumption must be explicitly stated in the guidelines, or the limits should be adapted to be intrinsically safe. In the absence of limitations applied to the peak-to-average power ratio of pulses, it is possible to deliver to the body large amounts of energy within a very short time interval. For millimeter-wave frequencies, where the absorption is superficial, this results in fast and dramatic temperature rises, as the step response function is proportional to the rapidly rising rather than the commonly encountered for deeper heating. As far as spatial averaging is concerned, it has been shown that an averaging area smaller than 4 cm2 should be introduced in order to avoid peak PDs in narrow beams [Neufeld and Kuster, 2018] that overheat the tissues. With increasing beam radius, e.g. at larger distances from the antenna(s), the tolerable averaging area increases rapidly, provided that there are no sharp exposure peaks. Duration-independent limits on the fluence of pulses are not suitable. They should either be replaced by duration-dependent fluence limits for pulses or by limits on the (temporal) peak exposure. In both cases, the limits should be set after taking narrow-beam exposures into consideration. These limits will depend on the chosen spatial and temporal averaging schemes and the maximum temperature increase deemed acceptable. Forward-looking knowledge about the technical needs and priorities of the industry could allow for selecting the balance between thresholds (averaging time and area, peak-to-average ratio, PD) to minimally impact the technological potential using the same limit-setting framework.

Machine Enhanced Logic

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 directionharm

PopulationNot supplied

Sample sizeNot supplied

ExposuremmWave · short pulses; pulses <1 s mentioned

Evidence strengthLowConfidence: 74% · Peer reviewed: yes

Main findings

The authors report that applying current and proposed exposure limits for pulsed fields could allow skin temperature increases up to about 10°C for short pulses at frequencies between 6 and 30 GHz. They also report that a proposed 4 cm^2 averaging area may not prevent high temperature increases for narrow beams; a Gaussian beam with 1 mm radius could yield ~10× higher temperature increase than a plane wave with the same averaged power density. They conclude that duration-independent pulse fluence limits may be insufficient and suggest duration-dependent limits and/or peak exposure limits that account for narrow-beam exposures.

Outcomes measured

skin temperature increase
tissue heating/overheating risk under guideline averaging schemes
power density spatial/time averaging implications

Limitations

No explicit study design details beyond an analysis/discussion are provided in the abstract/excerpts
Focuses exclusively on tissue temperature increase and does not consider other mechanisms (e.g., thermoelastic effects) per the excerpt
No human/animal sample described

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": "mmWave",
    "source": null,
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": "short pulses; pulses <1 s mentioned"
  },
  "population": null,
  "sample_size": null,
  "outcomes": [
    "skin temperature increase",
    "tissue heating/overheating risk under guideline averaging schemes",
    "power density spatial/time averaging implications"
  ],
  "main_findings": "The authors report that applying current and proposed exposure limits for pulsed fields could allow skin temperature increases up to about 10°C for short pulses at frequencies between 6 and 30 GHz. They also report that a proposed 4 cm^2 averaging area may not prevent high temperature increases for narrow beams; a Gaussian beam with 1 mm radius could yield ~10× higher temperature increase than a plane wave with the same averaged power density. They conclude that duration-independent pulse fluence limits may be insufficient and suggest duration-dependent limits and/or peak exposure limits that account for narrow-beam exposures.",
  "effect_direction": "harm",
  "limitations": [
    "No explicit study design details beyond an analysis/discussion are provided in the abstract/excerpts",
    "Focuses exclusively on tissue temperature increase and does not consider other mechanisms (e.g., thermoelastic effects) per the excerpt",
    "No human/animal sample described"
  ],
  "evidence_strength": "low",
  "confidence": 0.74,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "millimeter-wave",
    "above 6 GHz",
    "pulsed exposure",
    "localized exposure",
    "spatial averaging",
    "time averaging",
    "power density",
    "Gaussian beam",
    "skin temperature increase",
    "ICNIRP",
    "IEEE"
  ],
  "suggested_hubs": [
    {
      "slug": "who-icnirp",
      "weight": 0.86,
      "reason": "Discusses revised/approved ICNIRP guidelines and IEEE standard and whether limits meet heating-prevention goals for pulsed/localized mmWave exposure."
    },
    {
      "slug": "5g-policy",
      "weight": 0.72,
      "reason": "Addresses exposure limits and averaging schemes for frequencies above 6 GHz (mmWave), relevant to 5G/mmWave safety frameworks."
    }
  ]
}

AI can be wrong. Always verify against the paper.

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/bem.22244
PubMed: Not supplied

Read the original paper →

Related papers

← Back to the Research Library