If the orbit of a Near Earth Asteroid (NEA) is such that there is a (typically small) chance it will collide with the Earth and cause significant damage at some point in the future, it is also labelled a Potentially Hazardous Asteroid (PHA).
NEAs are classified as PHAs if their Minimum Orbit Intersection Distance (MOID; their closest possible approach to Earth) is less than 0.05 AU, and their absolute magnitude is less than 22 mag. Given that NEAs are thought to have albedos between 0.25 and 0.05, this magnitude translates to an asteroid size of 110 m to 240 m for the smallest PHAs. At this minimum size, the effects of the impact will be significant but mostly localised. However, by the time we reach kilometre-sized objects (magnitude ~ 18), areas the size of whole countries will be directly affected, and the impact will have global consequences.
Once discovered, PHAs are monitored continuously as their orbits may be perturbed through gravitational interactions with the planets and other bodies. These perturbations may significantly reduce the MOID of the asteroid over a short period of time and increase the risk of collision. Alternatively, the MOID may increase, essentially removing the hazardous threat of the object.
There are currently 719 PHAs catalogued, 154 of which have diameters of 1 km or greater.