Potential Ignition Sources which can carry enough
Energy to start an Explosion (acc. EN 1127-1):

• Hot surfaces
• Flames
• Sparks from electrical equipment
• Electrical equipment and lights
• Stray currents from electrical equipment
• Electrostatic discharge sparks
• Lightning strikes
• Electromagnetic fields in frequencies from 9kHz up to 300GHz
• Electromagnetic radiation in frequencies from 300GHz up to 3 x 106 GHz
  and of different wavelengths from 1000µm up to 0,1 µm (optical spectral range)
• Ionizing Radiation
• Ultrasonic
• Adiabatic compression, shock wave, running gases
• Chemical reaction
• And not to forget: the human factor which turns all potential
  ignition sources into effective ones.