Try to perform construction activities when you are not producing product and keep all your product contact surfaces covered. Try to also provide a dedicated entrance for contractors and cordon off the areas they're allowed to be in. If you cant....
.....create segregation with plastic sheeting between construction areas and product areas. Limit activities that have excessive dust or contamination potential to low risk periods. If you can't..
..Still provide shielding and take some additional environmental samples to try and quantify the risk, but this is a pretty bad situation. Ultimately some construction activities just require lines to go down.