Other than the absence of stops or guards, the plaintiff has not alleged or shown any physical defect in the window itself. Because the actual use of stops, even if installed, is optional with the tenant, we do not think that they reasonably can be regarded as vital to the use of the unit. Additionally, whether the absence of window guards, described in the plaintiff’s submissions on summary judgment as “metal bars attached to *451the outside of the building with one-way screws . . . [which] cover the bottom half of the window [and which] firefighters have little difficulty prying ... off with crowbars,” endangers or materially impairs the safety of an occupant of a unit to the extent they should be made mandatory is a matter we think best determined by the Legislature or the appropriate rule-making authorities rather than the courts.