Cotton threads that are mostly colored in red or black are commonly worn on arms, wrists or waist among people of all age groups. Very commonly these are used as a religious practice in Hindu religion. There exist non religious reasons for its use also.
There are various adverse effects of using these. It can cause allergic reactions that can lead to white skin called leucoderma or dark patches. These patches may be quite resistant to treatment and are usually long lasting.
These remain wet after bath for a long time and work as a nidus for fungal infection in the body. This fungal or sometimes bacterial infection may spread to distant areas. Despite complete treatment courses, some people may not get completely cured as the microorganisms remain safely here.
Thus, it should be remembered that faith in religious things is good but that should not be confused with unhealthy practice.
What should be done?
Avoid using these threads beyond a very short periods.
Consult a dermatologist immediately after you notice any skin problem around these threads.