"a level of activity" is grammatically correct and can be used in written English.
It is often used to describe the amount or degree of activity that someone or something is engaging in. Example: The students were overwhelmed by the high level of activity on campus during orientation week.
"We're still at a level of activity not seen since the '60's," she said.
One gram of radium-226 undergoes 3.7 × 1010 disintegrations per second, a level of activity that defined the curie (Ci), an early unit of radioactivity.
Mr. Bernson was responding to a warning by the Los Angeles police of the potential for "a level of activity" at least as intense as the violent protest that disrupted a meeting of the World Trade Organization in Seattle last December.
Foreigners have now been net buyers of Japanese stocks for 16 consecutive weeks, and trading at the Tokyo Stock Exchange has surpassed a billion shares a day for most of the spring and summer, a level of activity not witnessed since the 1980's.
But for now, at least, we're seeing a level of activity that was higher than 2016.
We note that the OFF state does not mean the complete absence of activity but a level of activity that is not sufficient to regulate target nodes.
Is there a level of Twitter activity that reflects political stability?
By Amy Davidson Sorkin March 9, 2011 Is there a level of Twitter activity that reflects political stability?
I, at the age of fifteen, was an adherent of this philosophy, though calling it philosophy would suggest a level of cerebral activity that just wasn't there.
It is a decline beginning at a lower level of activity than we had in the 1930s.
Scores range 0 100, higher scores represent a better function and a higher level of activity [ 19].
