Every civil jet which can operate at those altitudes is certified by the FAA (or equivalent authority) to FAA regulation FAR 34:
"SECTION: Sec. 34.31
Amendment Number: 34-5, Effective Date: 12/31/2012
TITLE: Standards for exhaust emissions.
SECTION RULE: (a) Exhaust emissions of smoke from each in-use aircraft gas turbine engine of Class T8, beginning February 1, 1974, shall not exceed a smoke number (SN) of 30.
(b) Exhaust emissions of smoke from each in-use aircraft gas turbine engine of Class TF and of rated output of 129 kN (29,000 lb) thrust or greater, beginning January l, 1976, shall not exceed
- SN=83.6(rO) −0.274 (rO is in kN).
(c) The standards set forth in paragraphs (a) and (b) of this section refer to exhaust smoke emission emitted during operation of the engine as specified in the applicable sections of subpart G of this part, and measured and calculated in accordance with the procedures set forth in subpart G."
So after a very small amount of smoke which may be emitted, the rest of what can be seen is water vapour - not any conspiracy armageddon chemical. Relax about it and enjoy life without that being a worry - your FAA has it covered for you!