rugged, crush resistant design with handle making it easy to transport,low permeation tank eliminates fuel evaporation and retains octane value longer constructed of sturdy seamless one-piece polyethylene plastic for durability. the swelling is a result of the fuel vapor building pressure, because the cap must stay sealed up to 5 psi. scepter tanks are built to last. caution: tank swelling will occur. epa regulations have eliminated the traditional venting in these tanks. equipped with scepter`s 2 way vented cap; this automatic air inflow valve system allows air to enter tank during operation and pressure to be released above 5 psi..syi