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California Landlord Can Be Liable for Toxic Mold Based Upon Expert Testimony

In California, plaintiffs have the burden of proving all of the elements of negligence when they file personal injury lawsuits. This includes presenting evidence that the defendant’s negligent conduct caused the plaintiff’s injuries and resulting losses. It is often necessary to present expert witness testimony to prove causation. When a defendant objects to a plaintiff’s witness being qualified to testify as an expert, the court must evaluate the witness’s testimony under the substantial factors standard. In Brancati v. Cachuma Village, Inc., Cal. Ct. App. Case No. B321616, the California Court of Appeal considered whether a trial court erred when it found that a plaintiff’s proposed expert witness was not qualified to testify about causation.

Factual and Procedural Background

Dana Brancati signed a month-to-month apartment lease with Cachuma Village, Inc. in 2012 and continued to live there from April 2012 to April 2016. While she lived there, she complained to her landlord about mold in her apartment and the company’s failure to remediate the issue. In 2016, Insight Environmental assessed Brancati’s apartment and found high levels of several species of toxic mold on her premises. She then filed a lawsuit against Chachuma Village, alleging the company had breached the warranty of habitability, constructively evicted her, caused her personal injuries, and committed fraud. Brancati attributed her respiratory illnesses to the toxic mold and claimed $500,000 in damages.

Brancati’s medical expert, Dr. Ronald Simon, M.D., testified during his deposition that Brancati’s respiratory illnesses began shortly after she moved into her apartment and that they could be attributed to the presence of high levels of toxic mold in her environment. Cachuma Villages filed a motion in limine to exclude Dr. Simon’s testimony, arguing that he was not qualified to testify as an expert about the medical causation of Brancati’s illnesses being from toxic mold.

In Brancati’s opposition, she argued that Dr. Simon was qualified as an expert and that his testimony was based both on his medical knowledge and on the scientific literature, which reflected that damp environments with toxic mold are a well-known cause of respiratory illnesses. The trial court held a hearing and found that Dr. Simon was not qualified as an expert on the medical causation of Brancati’s illnesses. Since Brancati couldn’t proceed to trial with his testimony, the trial court dismissed her case. Brancati filed an appeal.

Issue: Whether the court erred in finding the plaintiff’s expert witness was not qualified, thus preventing the plaintiff from taking her case to a trial?

The appeal asked the California Court of Appeal to determine whether the trial court erred when it granted the defendant’s motion in limine to disqualify Dr. Simon as an expert. The plaintiff argued that the court erred when it did so because the Dr. was qualified as an expert medical doctor and scientific researcher. When the court granted the motion in limine, it also prevented the plaintiff from having her case decided by a jury because she could not prove causation without her expert’s testimony.

Rule: Trial courts must ensure an expert witness’s testimony is based on sound reasoning and reliable material when determining whether they are qualified to testify as experts.

Trial courts play a gatekeeping role when determining whether a party’s expert witness is qualified to testify as an expert. Courts do not decide the probative value of an expert’s testimony. For evidence about causation, the court must evaluate it under the substantial factor standard.


Property owners have a duty to maintain their properties in a reasonably safe condition for people who are lawfully present. This includes property managers and landlords who must keep their premises in a reasonably safe condition for their tenants. If they breach this duty, they can be liable to pay damages in a premises liability lawsuit.

The Court of Appeal began by examining the role trial courts play when determining whether to qualify witnesses as experts. The court first noted that the court’s determination is not based on how persuasive the judge believes the expert’s opinion is but rather on whether it is based on reasoned logic. The court must look at the material relied on by the expert and determine whether it provides a logical basis for the expert’s opinion instead of the expert using conjecture or a leap in logic.

Trial courts must assess an expert who will be testifying about causation under the substantial factors standard. Under this standard, the factual basis of the expert’s testimony should be more than theoretical. The opposing party can challenge the factual basis of an expert’s testimony during cross-examination, and the testimony should only be excluded if it’s completely unsupported to the extent that it would offer no help to the jury. Under San Jose Neurospine v. Aetna Health of California, Inc. 45 Cal.App.5th 953 (2020), medical doctors are recognized as experts who are best positioned to determine the types of illnesses suffered by their patients.

In Brancati’s case, Dr. Simon opined that Brancati’s respiratory illness was caused by her living in an environment plagued by toxic mold and filled with mold spores. The trial court found that Dr. Simon was not qualified to determine her illness was caused by mold. However, the Court of Appeal found that Dr. Simon’s testimony was based on facts and did not amount to conjecture.

The 2016 report completed by Insight Environmental found that excessive levels of toxic mold were present in Brancati’s apartment, including aspergillus and penicillium, which were both located around Brancati’s shower. The air in the hallway showed high levels of aspergillus, penicillium, and a third type of mold called Stachybotrys chartarum. All three types produce fungal spores that can be toxic when inhaled. The report included photographs of mold growth in the apartment. Stachybotrys chartarum is a toxic mold that has been shown to have caused the death of animals. It has been found to be especially dangerous for young children, and health professionals have linked it to sudden infant death syndrome (SIDS).

The Court of Appeal then examined the ways that plaintiffs can prove that mold is the cause of an illness. First, a plaintiff can call an expert witness who can give an expert opinion based on a methodology accepted by the scientific community. The expert can also rely on epidemiological studies showing a statistical link between exposure to toxic mold and the development of respiratory illnesses.

The second method a doctor can use is a differential diagnosis of the patient. In this method, the doctor will include multiple potential diagnoses based on the patient’s symptoms and then conduct medical tests to rule them out until one is left.

In Brancati’s case, Dr. Simon relied on epidemiological studies showing a well-established link between toxic mold exposure and the development of respiratory illnesses. He also used a differential diagnosis to determine whether the mold in her apartment was the cause of her respiratory illness. As a medical doctor, Dr. Simon routinely used differential diagnoses to identify the source of patients’ illnesses and provide the most appropriate treatment. He was a board-certified medical doctor in immunology and allergies and was qualified to diagnose Brancati’s condition and its cause.


The Court of Appeal found that the trial court erred when it disqualified Dr. Simon as a medical expert who could testify about causation. It reversed the trial court’s decision to disqualify Dr. Simon and dismiss the case. The case was returned to the trial court for further proceedings consistent with the Court of Appeal’s decision, and Brancati was awarded her costs on appeal.

Talk to an Experienced Injury Lawyer

Exposure to toxic mold can result in serious medical illnesses. If you have developed respiratory conditions and believe your illness was caused by toxic mold in your rental home, you might have a viable claim against the property owner. Talk to an experienced injury attorney at the law firm of Steven M. Sweat, Personal Injury Lawyers, APC today by calling us for a free case evaluation at 866-966-5240.

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