I don't see why altruists can't pay for air conditioners to be installed in sweat shops.

1. Air conditioning has some cost associated with it; would the workers rather have the cost, or the cool air? It might be better to just give them the money, in which case it now competes with all other cash transfers for effectiveness.
2. This subsidizes the owners of sweatshops, which may have undesirable downstream effects, or set odd expectations.