Updated, and I will be adding to this post.
I was asked in a blog email about a specific product, RIT Sun Guard, now just called Sun Guard. I had never heard of it, so I checked it out.
There is a fair amount of information about it online, some of it conflicting on certain websites. The manufacturer's website has a fact page that seems fairly clear to me. Product formulations and information can be subject to change for a number of reasons and the manufacturer's website should have the most current information. If there are concerns regarding product safety, I think it is best to contact the manufacturer and ask for the current finished product MSDS if it is not available on their website. I did find such an MSDS dated 2005 elsewhere but that may not be most the current one and I have not listed it below.
From all I have read, the amount of SPF protection this product provides can be limited by how it is used, and the fabrics it is used on. There are specific directions to follow that apparently can be affected by the type of washing machine used too. I have not been able to read the caution on the front of the box on the lower left corner. I always read any cautions or warnings on product packaging very carefully. They are there for good reasons.
If I have any concern about a product and access to a manufacturer toll free contact telephone number or an email, I make use of either to ask the questions I need to ask and hopefully get the required information. I recommend doing that. Product reviews can be very informative. However, I want as much reputable, factual information as possible. I do not shy away from asking for links to such facts or research when I am curious while making inquiries to any source, to back up information. I have gotten such information in the past. See the Mayo Clinic link too, that has information on sun protection and clothing and on treated for SPF clothing.
I have always sought some sun protection as I do not tan. I am a great shade seeker when out in the hours between 10 am and 4 pm. I recently had to be out quite a bit during those hours. The tight weave, untreated for SPF clothing I wore that was still light enough to keep me cool, did not result in my very fair skin getting any colour. I also wore an untreated, tight weave, light coloured and light fabric, satin like kerchief to cover my hair (my new umbrella for sun protection would have been awkward to use because I was carrying packages). That worked just fine as well. My scalp did not get sunburned and my hair did not become lighter in colour or damaged.
"UPF rating indicates how effective a fabric is at blocking out solar ultraviolet radiation ... Factors that contribute to the UPF rating of a fabric are: Composition of the yarns (cotton, polyester, etc) Tightness of the weave or knit (tighter improves the rating) Colour (darker colours are generally better) Stretch (more stretch lowers the rating) Moisture (many fabrics have lower ratings when wet)"