Hi - I just went through this here in CA, and I can tell you what I did, as a heads up.
The mold we found was under the house and the tenants weren't complaining about feeling ill, but it still required us to handle it above board to avoid future claims of illness associated with the mold. We hired an "environmental hygenist" to test the mold and air to determine what the mold was and how significant. That report was then shared with the mold clean up company (those that test for mold are not the ones that clean it up, to avoid a conflict of interest) for them to do a visual inspection as well as a review of the report. The key was to stop the source of the mold, which in our case was two different leaks from the drain on the tub. Once we repaired those, the clean up company came in with all their hazmat suits, 24x7 air scrubbers, etc and got rid of the mold (kinda freaking out the tenants in the process). Then after they said they took care of it, the environment hygenist came back to provide certification that it was free of mold. While the air scrubbers were running (think loud fan), the tenants could not use the bedroom for a few days, so we compensated them with a $300 check and a thank you for enduring the interruption to their lives. Some cities under rent control have very specific daily amounts you must compensate the tenants, so best to check that out, as I was lucky this wasn't the case.
Overall, it cost about $3500 for the mold testing/abatement, not including the plumbing repairs and tenant compensation. My goal was to be sure it was safe for the tenants, and to have proof that it was all taken care of to avoid future health claims. I hope that helps give you an idea of what you may be in for. If your tenants are already complaining of health issues, you may want to consult with an attorney to do everything precisely by the book. So sorry you're having to deal with this. It's quite a journey!
