Describe the harmful effects on people of excessive exposure to electromagnetic radiation, including: (a) microwaves; internal heating of body cells (b) infrared; skin burns (c) ultraviolet; damage to surface cells and eyes, leading to skin cancer an

3.3 Electromagnetic Spectrum – Harmful Effects of Excessive Exposure

Cambridge IGCSE Physics (0625) – key syllabus points covered

• Identify the main health hazards of microwaves, infrared (IR), ultraviolet (UV), X‑rays and γ‑rays.
• Explain why microwaves, IR and UV are non‑ionising whereas X‑rays and γ‑rays are ionising.
• Give typical everyday and industrial sources for each region.
• State simple safety measures (including symbols and legal exposure limits) that reduce the risk of damage.
• Recall the basic wave‑property terminology (frequency \$f\$, wavelength \$\lambda\$, amplitude) and the relationship \$c = \lambda f\$ (with \$c = 3.0\times10^{8}\,\$m s⁻¹ in vacuum).
• Understand the photon‑energy threshold for ionisation (≈ 10 eV) and be able to calculate photon energy \$E = hf\$.

Basic Electromagnetic‑Wave Properties

• Speed in vacuum: \$c = 3.0\times10^{8}\,\$m s⁻¹ (the same for all EM waves).
• Frequency (\$f\$): number of wave cycles per second (Hz). Higher frequency ⇒ higher photon energy.
• Wavelength (\$\lambda\$): distance between successive crests (m). \$\lambda = c/f\$.
• Amplitude: related to the intensity (power per unit area) of the wave.
• Photon‑energy threshold for ionisation: \$E{\text{ion}} \approx 10\,\$eV. Using \$E = hf\$, the corresponding frequency is \$f{\text{ion}} = E{\text{ion}}/h \approx 2.4\times10^{15}\,\$Hz (≈ UV‑C). Waves with \$f > f{\text{ion}}\$ (X‑rays, γ‑rays) can remove tightly‑bound electrons from atoms.

Electromagnetic Spectrum – Order of Regions (by increasing frequency / decreasing wavelength)

1. Microwaves (≈ 1 mm – 1 m; 300 MHz – 300 GHz)

• Why non‑ionising: Photon energy \$< 10^{-1}\,\$eV – far below the 10 eV ionisation threshold.
• Interaction with the body: Water molecules have a permanent dipole; the alternating electric field forces them to rotate, converting EM energy into heat.
• Harmful effect: Internal heating of body cells. Deep tissue temperature can rise enough to cause burns, interfere with nerve function, or affect the cardiovascular system.
• Typical everyday & industrial sources (and key applications):

  • Microwave ovens (≈ 2.45 GHz) – cooking.
  • Radar (air‑traffic control, weather) – detection.
  • Mobile‑phone base stations, Wi‑Fi routers (2.4 GHz, 5 GHz) – communication.
  • Satellite communication links – broadcasting.
  • Industrial microwave dryers – material processing.

• Safety measures & legal limits:

  • Maintain a safe distance (≥ 1 m) from high‑power transmitters.
  • Use metal shielding (oven door, Faraday cage) to contain the radiation.
  • Never operate a microwave oven when empty – prevents reflected power spikes.
  • Exposure limit (ICNIRP, 2020): Power density ≤ 10 W m⁻² for the general public.
  • Safety symbol: the standard “microwave radiation” pictogram (a wave inside a rectangle).

2. Infrared (IR) (≈ 700 nm – 1 mm; 300 GHz – 430 THz)

• Why non‑ionising: Photon energy \$< 1\,\$eV – well below ionisation threshold.
• Interaction with the body: IR is strongly absorbed by the outer layers of skin and by the water in tissues, converting EM energy directly into heat.
• Harmful effect: Skin burns and superficial tissue damage from prolonged or intense exposure.
• Typical everyday & industrial sources (and key applications):

  • Electric heaters, ceramic heat panels – space heating.
  • Heat lamps (e.g., food‑warming, reptile enclosures) – thermal radiation.
  • Remote‑control transmitters (≈ 38 kHz carrier modulated by IR) – consumer electronics.
  • Thermal‑imaging cameras, night‑vision devices – detection.
  • Sunlight – largest natural IR component (≈ 50 % of solar energy).
  • Industrial IR dryers – paint and textile processing.

• Safety measures & legal limits:

  • Avoid direct, prolonged exposure to high‑temperature IR emitters.
  • Wear heat‑resistant gloves, aprons or protective clothing when working with IR heaters.
  • For very intense sources (e.g., industrial IR furnaces) use IR‑blocking goggles rated for the wavelength range.
  • Exposure limit (ICNIRP, 2020): Radiant heat flux ≤ 10 kW m⁻² for the general public.
  • Safety symbol: a wavy line with a heat symbol (often shown as a red‑orange wave).

3. Ultraviolet (UV) (≈ 10 nm – 400 nm; 750 THz – 30 PHz)

• Why non‑ionising (UV‑A & UV‑B) but chemically active: Photon energies 3–10 eV are below the ionisation threshold but high enough to break chemical bonds (e.g., in DNA).
• Why ionising (UV‑C): Photon energy > 10 eV; can eject electrons from atoms, similar to X‑rays.
• Interaction with the body: UV photons are absorbed in the outermost skin cells (epidermis) and the corneal surface of the eye.
• Harmful effects:

  • Sunburn (erythema) – acute damage to skin cells.
  • Premature skin ageing, DNA mutations → increased risk of skin cancers (melanoma, basal‑cell carcinoma, squamous‑cell carcinoma).
  • Eye damage – photokeratitis (“snow‑blindness”), cataracts, possible contribution to macular degeneration.

• Typical everyday & industrial sources (and key applications):

  • Sunlight – natural source (UV‑A 315‑400 nm, UV‑B 280‑315 nm, UV‑C 100‑280 nm, the latter largely filtered by the atmosphere).
  • Tanning beds – intense UV‑A/UV‑B for cosmetic tanning.
  • Fluorescent and compact‑fluorescent lamps – contain a small UV component.
  • Welding arcs and industrial curing lamps – strong UV‑B/UV‑C.
  • UV‑sterilisation devices (e.g., water purifiers, air disinfectors) – use UV‑C to destroy microorganisms.

• Safety measures & legal limits:

  • Apply broad‑spectrum sunscreen (SPF 30 + ) and re‑apply every two hours; choose “UVA‑UVB” labelled products.
  • Wear UV‑blocking sunglasses (CE‑marked) and protective clothing (wide‑brimmed hats, long sleeves).
  • Limit exposure during peak solar UV hours (10 am – 4 pm) and seek shade.
  • For artificial sources, use shielding glass or appropriate filters (e.g., polycarbonate for welding helmets).
  • Exposure limit (ICNIRP, 2020): Effective UV dose ≤ 30 J m⁻² (UV‑A) and ≤ 10 J m⁻² (UV‑B) for the general public per day.
  • Safety symbol: the “UV radiation” pictogram – a sun with three wavy lines.

4. X‑rays and Gamma (γ) Rays (≈ 0.01 nm – < 0.01 nm; > 30 PHz)

• Why ionising: Photon energies > 10 keV (X‑rays) up to several MeV (γ‑rays) far exceed the 10 eV threshold, easily ejecting tightly‑bound electrons.
• Interaction with the body: Ionisation along the track of the photon damages DNA, proteins and other cellular structures throughout the body.
• Harmful effects:

  • Cell mutation → increased risk of cancers (skin, lung, thyroid, leukaemia, etc.).
  • Genetic damage that may affect future generations.
  • Acute radiation syndrome at very high doses (nausea, vomiting, hair loss, bone‑marrow failure).

• Typical everyday & industrial sources (and key applications):

  • Medical imaging – X‑ray radiography, CT scans.
  • Radiotherapy – high‑dose X‑rays/γ‑rays for cancer treatment.
  • Industrial radiography – non‑destructive testing of welds, pipelines.
  • Security scanners – airport baggage scanners.
  • Radioactive isotopes (e.g., \$^{60}\$Co, \$^{137}\$Cs) – nuclear medicine, sterilisation.
  • Cosmic‑ray background – natural source of γ‑rays.

• Safety measures & legal limits:

  • Use lead (or equivalent) shielding around the source and for the operator.
  • Minimise exposure time and maximise distance – follow the “time, distance, shielding” rule.
  • Apply the ALARA principle (As Low As Reasonably Achievable).
  • Wear personal dosimeters where required; monitor cumulative dose.
  • Exposure limits (ICRP, 2022): 1 mSv yr⁻¹ for the general public; 20 mSv yr⁻¹ for occupational workers (averaged over 5 years).
  • Safety symbols: the trefoil radiation warning sign (three black blades on a yellow background) for ionising radiation.

Link to Energy Resources (Syllabus Supplement)

• Solar energy is harvested from the visible and UV portions of the spectrum (photovoltaic cells) and from the IR portion (solar thermal collectors).
• Microwave and radio‑frequency technologies are used in wireless power transmission and in the generation of useful energy from the Sun (e.g., microwave beaming concepts for space‑based solar power).
• Understanding the harmful effects helps explain why shielding, filtering and safe operating distances are essential when converting EM radiation into useful energy.

Summary Table – Harmful Effects & Safety