B737-900
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 263; Flight Crew Type 922
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Illness / Injury
General Physical Injury / Incapacitation
Aircraft
While performing the preflight inspection on this B737-900ER; some fluid dripped down my right arm while in the wheel well.  I could not determine where it came from or what type of liquid it was; so I did not think anything of it and continued my duties. During the flight; my arm turned red then welted up and started to burn. By the time I returned home that night; it was blistering. I was impossible for a doctor to tell exactly what kind of liquid it was; speculation is hydraulic fluid. My chemical burn has responded well to treatment and I will be okay to fly next week. The burn traversed the inside of my elbow joint and was difficult to bend so I used sick leave for 2 trips.  I am not looking for anything but am concerned as to what might have happened had this fluid gotten into my eyes. Not once in any training I have received at [company] have the dangers of any caustic fluids that may be encountered on the flight line been addressed; nor what the immediate response associated with that type of encounter should be.
B737-900ER First Officer reported coming into contact with leaking hydraulic fluid during a preflight inspection; resulting in a chemical burn.