Berms are also used as a method of environmental spill containment and liquid spill control. They are used for small operations for containment or protection from contamination. The containments were filled with inert nitrogen, which prevented hydrogen from burning in the containment. The size of the particles renders most containment and personal protective equipment ineffective. Strategies in the control of an outbreak are containment or suppression, and mitigation. In reality, containment and mitigation measures may be undertaken simultaneously. Valves on lines for standby systems penetrating containment are normally closed. A combination of both containment and mitigation measures may be undertaken at the same time. These structures act as a containment boom. The auxiliary building was outside the containment boundary. For details, see Containment and quarantine below. Gaseous oxygen is excluded from the reactions by physical containment. There are concerns over virus containment and cross species infection. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. There are concerns with using this approach regarding virus containment and cross species infection.