For wide-body Airbus aircraft exhaust velocity at idle thrust is 70 knots if you are “standing” abeam the tailplane. At takeoff thrust, exhaust velocity has reduced to 70 knots by about 1000 feet (300 m) behind the aircraft. Airbus uses the 70 knot criteria as a potentially damaging wind velocity.
For the BAe146 at takeoff thrust, standing directly behind the aircraft is thought to be similar to holding your girlfriend's hairdryer in your face but with an oily smell...