I see your point based on the fact that you are trying to hide Social security numbers. If someone knows you are using an SHA1HASH on the SSN to create a unique identifier, then can just generate a quick list of all SSN numbers, SHA1HASH them, then compare to automatically have the SSN of the person in the record. Even worse, they can pregenerate all these in a hash lookup table, and have a key of 1 hash for every SSN. This is called a hash lookup table, and more complex forms are called rainbow tables.
This is why a second feature of hashing was invented. It is called salting. Salting is basically this; you create a salt, then modify your data using the salt.
For instance, say you had the SSN 123-45-6789 . You could salt it with the string "MOONBEAM". Your new string for hashing is "123-45-6789MOONBEAM"
Now, even if someone knows that you are hashing the SSN to generate your unique ID, they still don't know the salt you will be using, and so are unable to derive the original SSN by pre-hashing a list of all SSNs and comparing to your ID. You however, can always take the user's SSN, use the salt, and rehash the SSN+SALT to see if the user SSN matches up with their ID.
Finally, if you use just 1 salt for everything, and keep it secret, instead of being able to see the salt, and generate the corresponding SSN by running SSN increments + salt 100 million times and picking the match, they have to do a lot more work to retrieve SSN. This is because the 100 million SSN numbers have a relatively low amount of entropy. (10^9 combinations). By adding your salt and keeping it secret, instead of just running
SHA1HASH(111-11-1111) -> check hash match
SHA1HASH(111-11-1112) -> check hash match
SHA1HASH(111-11-1113) -> check hash match
They would have to run
SHA1HASH(111-11-1111a) -> check hash match
SHA1HASH(111-11-1111b) -> check hash match
SHA1HASH(111-11-1111c) -> check hash match
...
SHA1HASH(111-11-1111azdfg) -> check hash match
SHA1HASH(111-11-1111azdfh) -> check hash match
....
SHA1HASH(111-11-1111zzzzzzzzzzzzzzzz) -> check hash match
SHA1HASH(111-11-1112a) -> check hash match
SHA1HASH(111-11-1112b) -> check hash match
.. and so on until they finally get to
SHA1HASH(123-45-6789MOONBEAM) -> check hash match
at which point they finally did manage to crack the SSN + SALT
They don't even know how many characters long your salt is
So that is 10^(number of characters of your salt) times more work for them to do just to get 1 SSN, let alone get the whole table.