I REGRET the necessity of writing this letter, simply because Mr Chris Parton (May 21) has chosen not to refer to my stated source before querying the information I gave in my letter published on May 19.

A copy has now been sent to him, and from it he will see that there is an existing EU system for measuring and comparing the EPI (Environmental Pollution Index) of any car. However, that system is not very accurate and the Swedish ''Rototest'' is much more effective and accurate. He will also see that the pollutants measured are CO2, HC, CO and NOX: lead is not included, although it will be present in the exhaust gas of 10-year-old cars.

As a consulting thermal engineer I am not in urgent need of Mr Parton's advice about the Carnot Cycle, particularly as his information is considerably out of date. Modern high-pressure generating situations are approaching a thermal efficiency of 40%, about double his figure.

It would also have been helpful if he had read my letter a little more carefully, where I responded to Dave Holladay's comments on the relative efficiency of cars and public transport. My letter pointed out that what the two types of transport do are so different as not to be strictly comparable, but some 20 million car owners have reached the only sensible conclusion.

Peter M Spinney,

Westerton Dean,

Mugdock,

Glasgow.

May 21.