The magnitude of an earthquake considered dangerous generally starts at about 5.0 or greater. Specifically:

• Earthquakes with magnitude 3.5 to 5.4 are often felt but rarely cause significant damage.
• Magnitudes under 6.0 can cause slight damage to well-designed buildings but potentially major damage to poorly constructed ones.
• Earthquakes with magnitude 6.1 to 6.9 can cause damage to poorly constructed buildings and other structures in populated areas.
• Magnitudes 7.0 to 7.9 are classified as major earthquakes causing serious damage over larger areas.
• Magnitudes 8.0 and above are considered great earthquakes with severe destruction and loss of life over extensive regions.

Dangerous impact typically becomes more likely starting at around magnitude 5.0, and more significant destruction tends to occur from magnitude 6.0 upwards, depending on construction quality and local conditions. The severity also depends on other factors such as depth and proximity to populated areas.