<b>This is directly from the IRS Publication 502 which discusses medical deductions:</b>
You can include in medical expenses amounts paid for transportation primarily for, and essential to, medical care.
You can include:
Bus, taxi, train, or plane fares or ambulance service,
Transportation expenses of a parent who must go with a child who needs medical care,
Transportation expenses of a nurse or other person who can give injections, medications, or other treatment required by a patient who is traveling to get medical care and is unable to travel alone, and
Transportation expenses for regular visits to see a mentally ill dependent, if these visits are recommended as a part of treatment.
Car expenses. You can include out-of-pocket expenses, such as the cost of gas and oil, when you use a car for medical reasons. You cannot include depreciation, insurance, general repair, or maintenance expenses.
If you do not want to use your actual expenses, for 2009 you can use the standard medical mileage rate of 24 cents a mile.
You can also include parking fees and tolls. You can add these fees and tolls to your medical expenses whether you use actual expenses or use the standard mileage rate.
<b>As for the HEPA filter etc... as long as you have the reciept that you bought it in 2009 and have the Dr. write a letter stating that it was on the recommendation of a Dr. for medical reasons, you may deduct it. Remember that all deductions for medical reasons need to meet the 7.5% of taxable income in order to be itemized, otherwise if you do not have enough it is better to take the standard deduction.
Hope this helps</b>