Hello, I can understand your concern for the hypersensitivity of skin in your husband at skin on left upper arm , upper left chest, upper left back, and even more sore at arm pit. You have not enumerated for how long your husband has this or sensitive skin on left upper arm , upper left chest, upper left back, and even more sore at arm pit. I also need to know any associated systemic problem like hypertension or increased blood sugar like diabetes or nay medications for the same. In your husband’s kind of illness at old age of 70 years it is most frequently seen due to deficiency of vitamin B 12. It is also common if he is not eating green vegetable or if he has alcohol intake. All these factors causes low absorption of the vitamin B complex. In my patients I generally go for the serum level of vitamin B 12. If found low, then I give this vitamin for 3 months of period. You can take further guidance from your family physician. All the best.