A mold problem at a rental property doesn't automatically trigger free rent for all tenants. Most of the time, landlords can avoid interruption in their rental income stream by:
When a landlord takes these measures, it is highly unlikely that tenants could claim (successfully) that they are entitled to not pay rent because of the mold issue.
That said, some tenants who are unhappy with their landlord's handling of a mold problem might decide to withhold some or all of their rent.
Sometimes, tenant self-help measures are legal and justified, but the legality of these tactics depends on each state’s tenant remedies laws. Many states allow tenants to take matters into their own hands when a landlord fails to provide safe, habitable housing or refuses to respond to repair requests. The two tenant strategies that are most frequently allowed by states are:
To take advantage of these self-help remedies, tenants must follow the procedures laid out by state law. For example, most states require tenants to notify their landlord in writing about a mold problem and give their landlord a reasonable amount of time to address it before they can self-help. Also, many states limit the amount of money that tenants can spend when they repair and deduct. For example, tenants might be able to deduct a $100 repair to a leaky faucet from their rent, but wouldn’t be able to deduct a $1500 water heater repair bill—instead, the tenants would have to sue their landlord to make the repair or reimburse them. Also, these self-help remedies are usually reserved for situations in which an apartment is uninhabitable or there is a serious habitability issue. A minor mold issue won't provide tenants with ample legal justification to stop paying rent. Finally, tenants are not entitled to use either self-help strategy when they (or their guests) created the conditions that led to the mold problem.
For more advice on dealing with mold at a rental unit or building, see the Nolo book Mold and Your Rental Property: A Landlord’s Prevention and Liability Guide, by Ron Leshnower.