How do heat illness claims affect the workers' comp experience mod?

Heat illness claims enter the experience rating formula as actual losses and remain for up to three policy years. Because most heat claims fall below the primary loss split threshold (approximately $17,500 per claim in most NCCI states), they count dollar-for-dollar in the primary loss calculation, which has the most direct influence on the mod. Two additional heat claims at $30,000 each can add approximately $35,000 in primary losses to the mod calculation, pushing the modifier measurably upward and holding it there for three years.

Does a heat illness prevention program lower the experience mod?

Not directly. The mod formula compares actual losses to expected losses. A prevention program lowers the mod only by reducing actual claims. Prevention programs that reduce claim frequency show up in the mod over time as fewer claims appear in the experience period. The improvement is real but arrives with a lag: claims from a bad summer take three years to fully age out of the experience window, while prevented claims from a good summer don't appear in the formula at all, they're visible only as the absence of claims that didn't occur.

Which construction classifications are most affected by heat illness claims?

Roofing, site preparation, exterior carpentry, and concrete work have the highest heat illness exposure because they involve sustained outdoor labor during peak temperature hours. NCCI research shows that claim frequency on days exceeding heat thresholds is concentrated in states where outdoor construction continues through summer, including Florida, Georgia, Alabama, and South Carolina. These states have extended heat seasons that produce more exposure hours than northern markets.

How does the primary loss split affect heat claim costs in the mod?

The NCCI experience rating formula divides each claim's cost into primary losses (up to approximately $17,500 per claim) and excess losses (above that threshold). Primary losses enter the mod dollar-for-dollar. Most heat illness claims fall below the split point and therefore count entirely as primary losses. A $30,000 heat claim contributes $17,500 in primary losses, the maximum primary weight. If the claim is a heat exhaustion case that cost $12,000, the entire $12,000 is primary. Both scenarios have outsized impact relative to what might seem like a modest claim amount.

Field ReportMay 2, 2026 · 4 min read

Heat Claims and the Mod: The Prevention Equation Most Contractors Calculate Wrong

WC claim frequency in construction rises roughly 10% on peak heat days. Two or three additional heat claims hold the mod elevated for three years. The prevention ROI runs differently than most contractors calculate.

By TraciThe Orson Group

Field Report

10%

WC claim frequency increase on peak heat days, construction

NCCI research

At a glance

NCCI research documents that WC claim frequency in construction rises roughly 10% on days when temperatures exceed peak heat thresholds. Two additional heat claims on a $3 million payroll roofing account, at $30,000 each, add $60,000 to actual losses and approximately $35,000 in additional primary losses to the experience rating calculation. That shift can push a mod from 1.00 toward 1.12, where it stays for three policy years unless the claims close significantly below reserved amounts.

The standard way to calculate the return on a heat illness prevention program is straightforward: add up the cost of shade structures, cooling stations, hydration supplies, and supervisory time. Compare that number to what a heat claim costs.

The problem with that calculation is that it stops at the claim. It doesn't account for how long the claim sits on the experience rating worksheet.

What NCCI's frequency data shows

NCCI research on weather-related WC claims documents that claim frequency in construction rises roughly 10% on days when temperatures exceed peak heat thresholds compared to moderate-temperature workdays. The effect is concentrated in the summer months and in states where outdoor construction continues through peak heat hours. Florida, Georgia, Alabama, and South Carolina all fall in that category.

For a contractor whose workforce is routinely exposed during June through August, the frequency increase isn't a single event. It accumulates across multiple summer days, across multiple crew members, over a three-month period. A workforce that would generate 20 lost-time claims in a mild summer may generate 22 in a hot one.

Those two additional claims don't disappear in September. They enter the experience rating window and stay for up to three policy years.

The three-year accumulation

The Experience Modification Rate formula compares actual losses to expected losses over a three-year experience period. When heat claims add to actual losses in a given year, that addition remains in the numerator for three consecutive mod calculations. The claim closed in October doesn't leave the worksheet until the policy year it was incurred drops out of the experience period.

A roofing contractor with $3 million in payroll might carry expected losses of $250,000 to $280,000, depending on state and classification. Two additional heat claims at $30,000 each add $60,000 in actual losses. Because both claims are below the primary loss split threshold (approximately $17,500 per claim for the current experience period in most NCCI states), each counts dollar-for-dollar in the primary loss calculation. That's $35,000 in additional primary losses, not $60,000, because the primary cap applies.

Still, $35,000 added to a primary loss base of roughly $120,000 is a 29% increase in actual primary losses. On a mod that started at 1.00, that single bad summer can push the modifier to approximately 1.10 to 1.12, depending on the specific formula inputs for that state and payroll tier.

For a contractor whose WC premium is $180,000, a mod of 1.10 costs $18,000 per year more than a mod of 1.00. Over three years, while the summer's claims age out of the window, the cost of two heat claims is not $60,000. It's closer to $54,000 in additional premium, compounded against whatever base rate movement the market applies in those three years.

Where the prevention calculation usually goes wrong

Contractors who look at heat prevention as a cost item typically compare the program expense to the average heat claim cost: $25,000, $35,000, depending on the claim. The comparison usually shows that preventing one claim pays for a year or two of prevention program costs.

What that calculation misses is the multiplier. A heat claim doesn't cost $35,000. It costs $35,000 in claim payment plus three years of elevated mod applied against the base rate. In a stable rate environment, that multiplier adds 40 to 60% to the apparent cost. In a rising-rate environment, the same mod point costs more in premium each year.

NCI's data also shows that claim frequency on hot days is not evenly distributed. Heat stroke, the severe end of the heat illness spectrum, is more common in the first and last weeks of summer than mid-season, when workers are more acclimatized. A prevention program that front-loads acclimatization protocols for the early weeks of heat season addresses the highest-frequency period first.

What the formula can't see

One thing the mod formula does not capture is averted claims. A contractor who prevented two heat claims in a given summer has better actual losses than a contractor who did not. That improvement is real, but it's invisible unless it shows up in the comparative frequency trend over time. Expected losses in the formula are set by NCCI based on industry-wide data, not on individual contractor prevention investment.

The only mechanism through which a heat prevention program improves the mod is by reducing actual claims. Prevention programs that work show up in fewer claims in subsequent experience periods, and those fewer claims lower the actual primary loss numerator relative to expected primary losses. The mod reflects the outcome, not the effort.

What an audit would check

An audit examines heat-related claims in the experience period against their current reserve status. Heat claims are frequently reserved at initial diagnosis values that assume worst-case outcome: hospitalization, extended medical management, potential permanent impairment. If a worker has recovered and returned to full duty, the reserve may still reflect the worst-case scenario. A claim reserved at $45,000 that settled for $18,000 creates a favorable development that improves actual losses once the claim closes. That development doesn't appear on the worksheet automatically, and carriers don't always adjust reserves downward as the case improves.

If you have open heat-related claims from the past three policy years, send us your NCCI worksheet and we'll review whether the reserved amounts track the actual recovery or the initial prognosis.