marathon oil corporation notes to consolidated financial statements operating lease rental expense was : ( in millions ) 2008 2007 2006 minimum rental ( a ) $ 245 $ 209 $ 172 .
by how much did net rental expense increase from 2006 to 2008?
( a ) excludes $ 5 million , $ 8 million and $ 9 million paid by united states steel in 2008 , 2007 and 2006 on assumed leases . 27 . contingencies and commitments we are the subject of , or party to , a number of pending or threatened legal actions , contingencies and commitments involving a variety of matters , including laws and regulations relating to the environment . certain of these matters are discussed below . the ultimate resolution of these contingencies could , individually or in the aggregate , be material to our consolidated financial statements . however , management believes that we will remain a viable and competitive enterprise even though it is possible that these contingencies could be resolved unfavorably . environmental matters 2013 we are subject to federal , state , local and foreign laws and regulations relating to the environment . these laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites . penalties may be imposed for noncompliance . at december 31 , 2008 and 2007 , accrued liabilities for remediation totaled $ 111 million and $ 108 million . it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties that may be imposed . receivables for recoverable costs from certain states , under programs to assist companies in clean-up efforts related to underground storage tanks at retail marketing outlets , were $ 60 and $ 66 million at december 31 , 2008 and 2007 . we are a defendant , along with other refining companies , in 20 cases arising in three states alleging damages for methyl tertiary-butyl ether ( 201cmtbe 201d ) contamination . we have also received seven toxic substances control act notice letters involving potential claims in two states . such notice letters are often followed by litigation . like the cases that were settled in 2008 , the remaining mtbe cases are consolidated in a multidistrict litigation in the southern district of new york for pretrial proceedings . nineteen of the remaining cases allege damages to water supply wells , similar to the damages claimed in the settled cases . in the other remaining case , the state of new jersey is seeking natural resources damages allegedly resulting from contamination of groundwater by mtbe . this is the only mtbe contamination case in which we are a defendant and natural resources damages are sought . we are vigorously defending these cases . we , along with a number of other defendants , have engaged in settlement discussions related to the majority of the cases in which we are a defendant . we do not expect our share of liability , if any , for the remaining cases to significantly impact our consolidated results of operations , financial position or cash flows . a lawsuit filed in the united states district court for the southern district of west virginia alleges that our catlettsburg , kentucky , refinery distributed contaminated gasoline to wholesalers and retailers for a period prior to august , 2003 , causing permanent damage to storage tanks , dispensers and related equipment , resulting in lost profits , business disruption and personal and real property damages . following the incident , we conducted remediation operations at affected facilities , and we deny that any permanent damages resulted from the incident . class action certification was granted in august 2007 . we have entered into a tentative settlement agreement in this case . notice of the proposed settlement has been sent to the class members . approval by the court after a fairness hearing is required before the settlement can be finalized . the fairness hearing is scheduled in the first quarter of 2009 . the proposed settlement will not significantly impact our consolidated results of operations , financial position or cash flows . guarantees 2013 we have provided certain guarantees , direct and indirect , of the indebtedness of other companies . under the terms of most of these guarantee arrangements , we would be required to perform should the guaranteed party fail to fulfill its obligations under the specified arrangements . in addition to these financial guarantees , we also have various performance guarantees related to specific agreements. .