In each of the four color blocks, shade one lettered cell in each row/column such that you can draw a single current of air through all unshaded squares in the puzzle (including those without letters).
The air current will form a closed loop, avoid all shaded squares, and visit all unshaded squares. The air current can only flow north, south, east, or west, and it can make a 90-degree turn as often as necessary. For each of the lettered squares the air current passes through, it must exit via the arrow.