Photographer's Formulary PyroCat HD is a high acutance developer, formulated by Sandy King as an alternative to other pyrogallol-based staining developers.
The advantages over PMK that Mr. King cites for his formula include an approximately 1/3 stop greater effective film speed, 10-15% shorter development times, more consistent staining action, lower toxicity, and no streaking or mottling with reduced agitation.
Catechol (pyrocatechin) has a high vapor pressure and it is a phenol. The high vapor pressure means that solid catechol evaporates readily. When you open a bottle containing solid catechol, you can smell it. Always store solid catechol in a tightly capped glass container. When mixing a solution containing catechol, work in a ventilated area. When catechol is in solution, its high vapor pressure is not a problem.