Securing Window A/C units - liability

[Donald M.]

As my unit does not have central A/C (which is very common in the Boston area, properties built long before A/C existed and very difficult to install based on some of the structures), I am going to allow for tenants to provide their own window A/C units. How do you deal with making sure they install it securely, as if one falls out it could seriously injure or kill someone. And what do I state in the lease regarding liability? 

When I have installed window units for myself (for bedrooms, not commercial ones), I always screw it into the window and put in a second bracket on the window frame to prevent it from opening. But I don't know if a tenant would be as careful.