operating interruptions (including leaks, explosions, fires, weather-related incidents, mechanical failure, unscheduled downtime, labor difficulties, transportation interruptions, spills and releases and other environmental risks);
changes in laws or regulations;
foreign currency exchange risks;
our ability to implement our business strategies; and
creditworthiness of our customers.
Many of such factors are beyond our ability to control or predict. Any of the factors, or a combination of these factors, could materially affect our future results of operations and the ultimate accuracy of the forward-looking statements. These forward-looking statements are not guarantees of our future performance, and our actual results and future developments may differ materially from those projected in the forward-looking statements. Management cautions against putting undue reliance on forward-looking statements or projecting any future results based on such statements or present or prior earnings levels. Every forward-looking statement speaks only as of the date of the particular statement, and we undertake no obligation to publicly update or revise any forward-looking statements.
Quantitative and Qualitative Disclosures about Market Risk
Commodity Price Risk
A substantial portion of our products and raw materials are commodities whose prices fluctuate as market supply and demand fundamentals change. Accordingly, product margins and the level of our profitability tend to fluctuate with changes in the business cycle. We try to protect against such instability through various business strategies. Our strategies include ethylene feedstock flexibility and moving downstream into the olefins and vinyls products where pricing is more stable. We use derivative instruments in certain instances to reduce price volatility risk on feedstocks and products. Based on our open derivative positions at March 31, 2018, a hypothetical $0.10 increase in the price of a gallon of ethane would have increased our income before taxes by $2 million.
Interest Rate Risk
We are exposed to interest rate risk with respect to fixed and variable rate debt. At March 31, 2018, we had $11 million principal amount of variable rate debt outstanding. The debt outstanding under the tax-exempt waste disposal revenue bonds is at a variable rate. We do not currently hedge our variable interest rate debt, but we may do so in the future. The average variable interest rate for our variable rate debt of $11 million as of March 31, 2018 was 1.7%. A hypothetical 100 basis point increase in the average interest rate on our variable rate debt would not result in a material change in the interest expense. Also, at March 31, 2018, we had $3,154 million aggregate principal amount of fixed rate debt. We are subject to the risk of higher interest cost if and when this debt is refinanced. If interest rates were 1.0% higher at the time of refinancing, our annual interest expense would increase by approximately $32 million.
Foreign Currency Exchange Rate Risk
We are exposed to foreign currency exchange rate risk associated with our international operations. However, the effect of fluctuations in foreign currency exchange rates caused by our international operations has not had a material impact on our overall operating results. From time to time, we may engage in activities to mitigate our exposure to foreign currency exchange risk in certain instances through the use of currency exchange derivative instruments, including forward exchange contracts, cross-currency swaps or spot purchases. A forward exchange contract obligates us to exchange predetermined amounts of specified currencies at a stated exchange rate on a stated date. A cross-currency swap obligates us to make periodic payments in the local currency and receive periodic payments in our functional currency based on the notional amount of the instrument. In January 2018, we entered into hedging arrangements designated as net investment hedges with an aggregate notional value of 220 million euros to reduce the volatility in stockholders' equity from changes in currency exchange rates associated with our net investments in foreign operations. See Note 7 for additional discussion related to these net investment hedges.