How Much Hair may a Married Woman Expose?

Saying Shema in Front of a Woman’s Uncovered Hair

In addition to the basic principle of tznius (modesty), which is that both men and women must keep the private areas of their bodies covered at all times, even b’chadrei chadarim, when alone in private, because Hashem is always present, there are also particular parts of a woman’s body which are considered intimate and are therefore in the category of ervah, and must also be covered.

One example of the second category is sei’ar ba’isha ervah, a married woman’s hair is considered ervah, and when exposed, beyond being inherently problematic, also prohibits men from being able to say Shema, brachos, daven or learn Torah in its presence. This is true if even a single hair is exposed; the notion that a woman may allow up to a tefach of hair to be uncovered is groundless and comes from a misunderstanding of halachah. The only exception is that a husband may say Shema in the presence of his wife’s facial hair which grows beneath the area of her head covering, in front of her ears. 

Therefore, if your Shabbos guests are not properly dressed, you should either face a different direction, hold up your siddur to block your view, or close your eyes—the last of these options being the least preferred. #492

https://halacha2go.com?number=492

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.