Arizona Workers' Compensation: Understanding the "One Percent" Medical Causation Rule

Navigating a workers' compensation claim in Arizona requires a clear understanding of medical causation, particularly when a workplace event isn't the only cause of an injury. Arizona law is uniquely favorable to workers, employing what is often called the "One Percent Rule" for establishing that an injury is work-related.

What is Medical Causation?

In the Arizona workers' compensation system, a claimant must prove both legal and medical causation. While legal causation determines if an accident happened "in the course of" work, medical causation requires expert testimony to prove a workplace event caused the specific physical or mental condition. This medical opinion must be expressed in terms of probabilities rather than mere possibilities to be legally sufficient.

The "One Percent" Rule Explained

The foundation of Arizona’s causation standard is found in A.R.S. § 23-1021(A), which mandates coverage for injuries caused "in whole, or in part, or contributed to" by the employment.

• Any Contribution is Enough: Arizona courts, such as in Tatman v. Provincial Homes, have established that a work event does not need to be the sole or primary cause of an injury.

• The 1% Standard: If a work-related activity contributes even one percent to the overall medical condition, the injury is generally compensable.

• The "As He Finds Him" Principle: Rooted in the case of Murray v. Industrial Commission, this legal doctrine states that an employer takes the employee as they find them, including any preexisting vulnerabilities or conditions.

• Mixed Risk Cases: In "mixed risk" scenarios where personal health and work factors combine, the law simply asks if the work was a contributing factor (Martinez v. Industrial Commission).

Aggravation vs. Exacerbation

A critical distinction in these claims is whether a work event caused a permanent aggravation or a temporary exacerbation (flare-up) of a preexisting condition.

1. Temporary Exacerbation: This is a symptomatic flare-up where a condition temporarily worsens but eventually returns to its pre-injury "baseline". In these cases, the insurance carrier is typically only responsible for the medical treatment of the flare-up itself.

2. Permanent Aggravation: This occurs when a work event permanently worsens the underlying condition or triggers a new need for surgery. To establish a permanent disability, a claimant must show that the aggravation continues to contribute to their symptoms or functional loss (Arellano v. Industrial Commission).

When a Higher Standard Applies

It is important to note that the "one percent" standard does not apply to all types of claims. Under A.R.S. § 23-1043.01, certain injuries require a "substantial contributing cause" standard:

• Mental Injuries: PTSD or other mental illnesses not resulting from physical trauma must be caused by "unexpected, unusual or extraordinary stress".

• Heart-Related Injuries: Heart attacks or perivascular diseases are only covered if work exertion was a substantial factor.

The Importance of Objective Evidence

For a claim to move beyond temporary benefits to a permanent award or a successful reopening, there must be a change in objective physical findings.

Conclusion If you have a preexisting condition that was worsened by work, Arizona law provides broad protections. As long as your job contributed even slightly to your current medical need, you may be entitled to full workers' compensation benefits.