Bill Tuthill wrote:
Mike Romain wrote:

Does anybody find that there is a huge difference in effectiveness
between SPF 15 versus 40 sunblock?

One allows 1/15 of the UVB to pass, the other allows 1/40 of the UVB.

Yes, a huge difference.
I live in Canada and we are affected by the ozone hole here.
The local weather and newspapers give the ozone index every day.

The index is given everywhere. I don't know if the northern hole
affects anywhere but northern Canada--does anyone have a map of the
approximate dimensions?

Usually I wear clothing, which depending on fabric, works very well.
One day rafting I used SPF 40 instead, reapplying every time my legs
got wet, which was about every hour. Got a mild sunburn.

Some fabrics are surprisingly low, such as the old dress shirts that I
often use as a top layer in the sun or bugs.

Does the expiration date make a large difference in the efficacy of
certain sunblock chemicals? This SPF 40 was about a year past expiry,
although it contained titanium dioxide and zinc oxide, two ingredients
that I doubt degrade rapidly.

You are probably right. We just use the stuff until it is done. The
older bottles get sed up as daily kids-wear at home and/or in the car.

GaryJ