Attributable risk percent (AR%) is the proportion of disease cases among exposed individuals that would be eliminated if the exposure were removed. It answers a practical prevention question:
Of all leptospirosis cases that occurred in farmers who waded without boots, how many were actually caused by that specific exposure?
Step 1 — Calculate incidence in each group
The cohort data provide two incidence rates that must be computed before any comparison.
| Group | Cases | Total farmers | Incidence |
|---|
| Exposed (waded without boots) | 24 | 400 | 0.06 (6.0%) |
| Unexposed (did not wade without boots) | 6 | 600 | 0.01 (1.0%) |
Step 2 — Apply the attributable risk percent formula
The formula subtracts the baseline risk seen in the unexposed group from the risk in the exposed group, then expresses that difference as a proportion of the exposed group’s total risk.
AR% = (incidence in exposed − incidence in unexposed) ÷ incidence in exposed × 100
AR% = (
0.06 −
0.01) ÷
0.06 × 100
AR% =
0.05 ÷
0.06 × 100 =
83.3%
Step 3 — Interpret the result
An AR% of 83.3% means that about 83 out of every 100 leptospirosis cases among farmers who waded without boots can be attributed to that exposure. Equivalently, roughly five of every six cases in the exposed group would be expected to disappear if boot use during wading were fully implemented and effective.
Watch out! Attributable risk percent is not the same as relative risk or the overall proportion of cases in the population. AR% applies only to the exposed group and reflects the
preventable fraction within that specific subgroup.
Key point! The unexposed group still had a 1.0% incidence, which represents the background risk from other routes of
Leptospira exposure in a rice-farming environment — contaminated soil, water contact during other activities, or peridomestic animal reservoirs. The AR% calculation removes that background risk before estimating the effect of wading without boots.
Why this matters for leptospirosis prevention planning
Leptospirosis is a zoonosis transmitted through contact between broken skin or mucous membranes and water or soil contaminated by urine from infected reservoir animals, particularly rodents. Occupational exposure in agricultural settings is a well-documented risk factor because farmers routinely enter flooded fields, irrigation canals, and muddy terrain where
Leptospira can survive for weeks to months in warm, moist conditions . A case-control study in northern Iran similarly identified agricultural work and contact with surface water as significant risk factors for infection . Population-based surveillance data from the same region confirm that occupational and environmental exposures drive a substantial portion of the disease burden . Longitudinal surveillance in Fujian, China further links leptospirosis incidence to agricultural practices and reservoir animal activity, reinforcing the importance of targeting modifiable occupational exposures .
For the public health nurse planning prevention with the farmers’ association, the AR% value provides a concrete, evidence-based message:
the majority of leptospirosis cases in farmers who wade without protective footwear are attributable to that single modifiable behavior. This supports prioritizing boot distribution, subsidized protective equipment, and education on avoiding bare-skin water contact during field work. The calculation also demonstrates why interventions aimed at the exposed group — rather than the entire farming population — yield the greatest reduction in cases per unit of effort.
Common calculation errors to avoid
One frequent mistake is dividing by the total population instead of the exposed group’s incidence. Another is subtracting incidence rates incorrectly or forgetting to multiply by 100. The answer
16.7% arises from dividing the difference by the unexposed incidence, while
75.0% results from using the relative risk reduction formula incorrectly. Only
83.3% follows the standard AR% equation and matches the interpretation that five of six exposed cases are preventable by removing the exposure.