Passengers on a direct flight from Brussels to the United States are subject to a double security screening: apart from the usual security screening which applies to all passengers, they will be screened a second time at the gate before boarding.
Moreover, these passengers are allowed only one piece of carry-on baggage.

As a result of these measures, US-bound passengers are requested to present themselves at the check-in desk at least three hours prior to their departure time.

For passengers travelling to destinations outside the USA, no extra security measures apply.