I hear COVID-19 affects your breathing, can I use my iPhone to measure my oxygen levels?

No.  A pulse oximeter requires the comparison of readings from *two wavelengths of light to arrive at an oxygen saturation measurement.  iPhones have only one, and results from apps claiming to accurately determine this should not be trusted.  

The Centre for Evidence-Based Medicine says: There is no evidence that any smartphone technology is accurate for the measurement of blood oxygen saturation.  The scientific basis of such technologies is questionable.  Oxygen saturation levels obtained from such technologies should not be trusted.

(Sky Pittson, MD, April 27, 2020)