1 gradual increase
ADJ. gradual, slow | steady | rapid | massive VERB + BUILD-UP cause, lead to The leak led to a slow build-up of carbon dioxide. | avoid, prevent
PREP. during/in the ~ | ~ of
2 period of time before an event
ADJ. pre-match, pre-trial, etc.
PREP. ~ to Tension is mounting in the build-up to the elections.