diff --git a/src/utility/fan_policy.cpp b/src/utility/fan_policy.cpp
index 39983a9..293e3bc 100644
--- a/src/utility/fan_policy.cpp
+++ b/src/utility/fan_policy.cpp
@@ -21,7 +21,7 @@ constexpr bool policy_voc_improving(VOCIndex voc_improve_min, VOCIndex intake, V
     if (intake == NOT_KNOWN || exhaust == NOT_KNOWN) return false;  // Need a reading for both sensors.
 
     auto const voc_improvement = intake.value_or(0) - exhaust.value_or(0);
-    return voc_improve_min <= voc_improvement;
+    return 0 <= voc_improvement && voc_improve_min <= voc_improvement;
 }
 
 constexpr bool should_filter(FanPolicyEnvironmental const& params, VOCIndex intake, VOCIndex exhaust) {
@@ -80,5 +80,7 @@ static_assert(!policy_voc_improving(2, 3, NOT_KNOWN));          // basic case -
 static_assert(!policy_voc_improving(2, NOT_KNOWN, 1));          // basic case - need both sensors
 static_assert(!policy_voc_improving(NOT_KNOWN, 3, 1));          // disabled
 static_assert(!policy_voc_improving(2, NOT_KNOWN, NOT_KNOWN));  // sensors not connected
+static_assert(!policy_voc_improving(NOT_KNOWN, 68, 76));        // disabled, unusual intake/exhuast
+static_assert(!policy_voc_improving(1, 68, 76));                // disabled, unusual intake/exhuast
 
 }  // namespace nevermore