absence : 欠席、不在

〈U〉〈C〉『るす』,不在;『欠席』;不在期間;(…を)欠席すること《+『from』+『名』》 / 〈U〉(…が)『ないこと』,(…の)欠如(lacking)《+『of』+『名』》
The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities.