Hazard Level

Hazard Level (ハザードレベル) is a term coined by Faust to measure a human's resistance to Nebula Gas. A low Hazard Level means that a certain individual would have a much lesser chance of withstanding the gas' effects and have greater chances of dying. Higher Hazard Levels would allow a certain individuals to perform feats that would normally require a prerequisite Hazard Level.

Description
Individuals with Hazard Level 1 would die a short while after being administered with the gas, while individuals with Hazard Level 2 would immediately be converted into a Smash. Anyone with a Hazard Level above Level 2 would retain their human form even after being administered with the gas, with prime examples being Sento Kiryu and Ryuga Banjou. According to Gentoku and Takumi Katsuragi's data, anyone with a Hazard Level above Level 3 would be able to use the Transteam Gun or Build Driver, while those above level 4 are able to use a Sclash Driver.

Evidently, Faust's experiments were to isolate people with Hazard Levels high enough to use the Rider System so they could create an army of Kamen Riders for military purposes.

Theoretically, the Hazard Level of humans may be raised through two means: temporarily through strong emotions, and permanently through the continual usage of purified Fullbottles and the Build System.

Classification
The first Hazard Level mentioned in the series was Kasumi Ogura's. Due to her unnamed illness, her physical state could not withstand the transformation of being turned into a Smash. Night Rogue labeled her as "Hazard Level 1", claiming that once anyone extracted her Smash Essence, she would die immediately. Ryuga bitterly allowed Sento to extract her Smash essence, thus killing her. Her Smash essence was later purified into the Dragon FullBottle, which Ryuga treasured greatly.

Sento Kiryu/Kamen Rider Build is given Hazard Level 3.2 and later 3.7 by Blood Stalk, who notes that anger leads to his Hazard Level increasing.

While fighting Ryuga, Blood Stalk calls him Hazard Level 2.4, which grows incrementally to 2.7 throughout the fight. Later, due to Ryuga's determination and strong synchronization with the Dragon Fullbottle, his Hazard Level rose to 3.0, thus allowing him to become Kamen Rider Cross-Z.

Ryuga's Hazard Level later manages to get pushed up to a 4.0, allowing him to use the Sclash Driver.