Are static variables thread-safe? C#

I want to create a class which stores DataTables, this will prevent my application to import a list of details each time I want to retrieve it. Therefore this should be done once, I believe that the following code does so, but I am not sure if it is thread-safe.

The below code is in the Business Layer Section of my three tier application, it is returning a DataTable to the Presentation Layer.

public class BusinessLayerHandler
{
    public static DataTable unitTable;
    public static DataTable currencyTable;

    public static DataTable GetUnitList()
    {
        //import lists each time the application is run
        unitTable = null;
        if (unitTable == null)
        {
            return unitTable = DatabaseHandler.GetUnitList();
        }
        else
        {
            return unitTable;
        }
    }

    public static DataTable GetCurrencyList()
    {
        //import lists each time the application is run
        currencyTable = null;
        if (currencyTable == null)
        {
            return currencyTable = DatabaseHandler.GetCurrencyList();
        }
        else
        {
            return currencyTable;
        }
    }

Any help is appreciated, if there is a better way how to cache a DataTable please let me know.

Update:

Thanks to your opinions, this is the suggested method to do it, if I understood correctly:

public class BusinessLayerHandler
{
    private static DataTable unitTable;
    private static DataTable currencyTable;

    private static readonly object unitTableLock = new object();
    private static readonly object currencyTableLock = new object();

    public static DataTable GetUnitList()
    {
        //import lists each time the application is run
        //unitTable = null;

        lock (unitTableLock)
        {
            if (unitTable == null)   
            {
                return unitTable = DatabaseHandler.GetUnitList();
            }
        }
        return unitTable;
    }

    public static DataTable GetCurrencyList()
    {
        //import lists each time the application is run
        lock (currencyTableLock)
        {
            if (currencyTable == null)
            {
                return currencyTable = DatabaseHandler.GetCurrencyList();
            }
        }
        return currencyTable;
    }
}