## Facts

- In August 2004, police in Del Rey Oaks, California encountered Michael Rosa acting erratically; officers believed he was intoxicated or mentally unstable.
- During a prolonged struggle, Michael fled, damaged property, armed himself with a piece of lumber, and swung it at officers.
- Officers used TASER International’s ADVANCED TASER M26 electronic control device multiple times in probe mode to subdue him.
- After he was restrained, Michael stopped breathing and died.
- The autopsy identified ventricular arrhythmia due to methamphetamine intoxication as the cause of death, listing TASER application and arrest by police as contributing conditions.
- The plaintiffs (Michael’s parents and daughter) alleged that repeated TASER exposure can contribute to metabolic acidosis (lactic acid buildup lowering blood pH), increasing the risk of fatal arrhythmia.
- The M26 at issue was distributed in December 2003; TASER later issued a 2009 warning addressing metabolic acidosis and repeated exposures.
- Plaintiffs sued under California products-liability law, asserting strict-liability and negligence failure-to-warn theories based on the alleged risk of metabolic acidosis from repeated use.
- The district court granted summary judgment to TASER, finding the asserted risk was not known or knowable at the time of distribution; plaintiffs appealed.

## Issues

1. Under California failure-to-warn law, did TASER have a duty in 2003–2004 to warn that repeated M26 applications could lead to fatal metabolic acidosis?
2. Was that specific risk “known or knowable” in light of the generally recognized and prevailing best scientific and medical knowledge at the time of manufacture and distribution, such that plaintiffs could proceed on strict-liability or negligence failure-to-warn claims?

## Decision

- The Ninth Circuit affirmed summary judgment for TASER.
- The court held that plaintiffs failed to show the alleged risk—fatal metabolic acidosis from repeated M26 applications—was known or knowable in 2003–2004 under the prevailing scientific and medical knowledge standard.
- The court rejected plaintiffs’ reliance on expert opinions that extrapolated from general physiology and scattered reports without demonstrating contemporaneous, generally recognized knowledge of the specific hazard.
- The court treated TASER’s later warning as not determinative of what was knowable at the time of distribution.

## Legal Principles

- Under California law, a manufacturer’s duty to warn is limited to risks that were “known or knowable in light of the generally recognized and prevailing best scientific and medical knowledge” available at the time of manufacture and distribution.
- A failure-to-warn claim (in strict liability or negligence) requires evidence that the specific risk was reasonably scientifically knowable at the relevant time; general awareness of related physiological risks is insufficient.
- Expert testimony must be supported by reliable data and methodology; speculative or under-supported opinions do not create a triable issue of fact at summary judgment.
- Post-distribution warnings or later-developed scientific understanding do not retroactively establish that a risk was knowable when the product was distributed.

## Conclusion

The Ninth Circuit held that California’s state-of-the-art knowledge standard barred liability because plaintiffs could not show that, at the time the M26 was distributed and used, it was scientifically knowable that repeated discharges could cause fatal metabolic acidosis requiring a warning, so TASER was entitled to summary judgment on both strict-liability and negligence failure-to-warn claims.


Rosa v. Taser Int'l, Inc., 684 F.3d 941 (9th Cir. 2012)