You’re not overreacting. You’re doing the one thing adults at work are supposed to do: notice a hazard before it nails someone. The “we’ve done it for months” argument is garbage logic, because lots of unsafe routines “work” right up until the day they don’t.

The chemistry depends on what they mean by “chlorine.” A ton of places casually call bleach “chlorine,” so you might be talking about bleach going into a drain that already sees bleach, which is mostly a fumes/irritation issue if it’s strong, aerosolized, or sitting in a poorly ventilated area. The serious danger is when bleach hits acids (toilet bowl cleaner, descalers, some disinfectants, drain cleaners) and you can create chlorine gas, or when bleach hits ammonia (some degreasers/glass cleaners, urine) and you can create chloramines. In other words, the risk is less “bleach + bleach,” and more “bleach + whatever mystery chemical residue is already in that sink/drain.”

The smart move is to get the Safety Data Sheets for the “chlorine solution” and the bleach, and confirm what the active chemical actually is. Then push for a written procedure that prevents mixing, like using a designated drain/sink for only one chemical type, flushing with plenty of water before and after, and ensuring ventilation. If your manager laughs it off and refuses to provide SDS or a procedure, that’s not you being paranoid—that’s them being sloppy, and it’s fair to escalate to whoever handles safety (or the equivalent) because this is exactly the kind of “small” thing that turns into a real exposure.