logoalt Hacker News

piguin • yesterday at 10:40 PM • 1 reply • view on HN

The arms represent very little surface area: https://wikem.org/wiki/Estimating_body_surface_area

A critical factor for doing a screening is how many bad burns you've had on the chest and back which would probably be similar to many times the burns on the arms in terms of surface area x added probability.

Replies

pazimzadeh • yesterday at 11:23 PM

I like the idea of checking for that, but not sure the numbers add up.

Anterior head + legs + arms = 31.5% Anterior + Posterior torsos = 36%

> which would probably be similar to how many times the burns on the arms in terms of surface area x added probability

I disagree, my whole point is that frequent low/moderate exposure (such as arms) reduces the chance of burning during high exposure

I looked up the number, and after normalizing by surface area it looks like the skew towards torso is only for basal cell carcinoma (BCC) not (SCC).

https://jamanetwork.com/journals/jamadermatology/fullarticle...

For melanoma the trunk was the dominant site in men, while the legs were the dominant site in women

https://pmc.ncbi.nlm.nih.gov/articles/PMC2939095/

Unfortunately, that I know of there are not many studies that compare 'frequent low/moderate exposure followed by high exposure' vs 'very low exposure (high sunscreen use) followed by high exposure'

Here is what I found:

1. https://pubmed.ncbi.nlm.nih.gov/9335442/

  These results show the specificity of the positive association between melanoma risk and intermittent sun exposure, in contrast to a reduced risk with high levels of occupational exposure.
2. https://www.nature.com/articles/s41598-026-48477-4

  Both the low-intermittent (HR = 1.26; 95%CI = 1.17–1.38) and high exposure groups (HR = 1.13; 95%CI = 1.01–1.26) had higher rates of melanoma than indoor workers. 

  The low-intermittent exposure group had elevated melanoma rates on the arms and trunk (body), while the high exposure group had an elevated rate on the head and trunk.
Digging into the data, the moderate exposure group had no detectable increase at any body site and a weak suggestion of lower risk (not statistically significant):

  Head: 0.99; Arms: 0.95; Trunk: 1.00; Legs: 0.90
Plotted: https://imgur.com/a/HBp8SnH

Why would the low-exposure group have a higher rate on the arms and trunk? Could it be because when they do go to the pool/beach, their arms are not protected? Whereas in the high exposure group, their arms are protected due to everyday exposure, but their trunk is only exposed when they go to the pool? Then again, you would think the low-exposure group would also have high trunk incidence, so there might be some confounding factors like ethnicity/behavior/dress.

Anyway, hopefully I convinced you that it's not as simple as more sun = always bad.