The best way to keep T-shirts white is to treat stains and wash them only with other white clothing within 24 hours of usage. T-shirt fabric yellows or grows dingy for a variety of reasons including age and improper washing -- too little detergent in the washing machine, wrong water temperature, inadequate rinse cycle and being washed with dark clothing. Laundry bleach will effectively whiten your T-shirt, but there are many natural cleaners that will get that shirt white again.
Laundry bleach is available in powder or liquid forms. Modern bleach is available for coloured and white clothes and will be clearly marked as to which product is safe for which kinds of clothing. The University of Illinois Extension states that bleach intended for both white and coloured clothing takes longer to whiten fabric than chlorine bleach intended for whites only.
Lemon Juice and Sunshine
Lemon juice is a natural citrus cleaner. The acidic properties of lemon juice make it unsafe to mix with bleach, and toxic fumes will result if the two are mixed. Lemon juice is a natural whitener and when combined with the power of the sun, will naturally bleach your T-shirt. Add about 1/2 cup of lemon juice to the washing machine water along with your usual detergent (as long as it contains no bleach ingredients). Wash the shirt as usual. Hang the shirt on a clothesline in the sunshine until dry.
Baking soda, also called sodium bicarbonate or bicarbonate of soda, is a naturally and synthetically produced chemical compound, often found in the mineral natron. Baking soda is an effective cleaner by itself, or it can be mixed with lemon juice or vinegar to release carbon dioxide. The release of carbon dioxide creates forceful bubbles that aid in cleaning. Soak yellow or greying T-shirts in a container with water and 1 cup of baking soda overnight, or add 1 cup of baking soda to a small load in the washing machine and wash. Wash again with your normal clothes detergent. Baking soda also works as a fabric softener.
Vinegar is an acidic liquid obtained during the fermentation process of alcoholic liquids. Vinegar is an effective cleaner by itself or when combined with other properties. Vinegar should not be mixed with bleach or products containing bleach or harmful fumes will result. Use vinegar with caution when doing the laundry. The University of Illinois Extension warns that undiluted white vinegar may weaken and stress cotton fabrics, such as those used in T-shirts. Vinegar is more effective as a stain treatment, which should then followed with clothes detergent in the wash machine. Add 1 cup white vinegar for every gallon of water used in a bucket or washing machine.
Hydrogen Peroxide or Borax
Hydrogen peroxide is naturally occurring mild bleach. Hydrogen peroxide will effectively remove stains or whiten a white T-shirt. Use 1 cup of 3-percent hydrogen peroxide to a small-to-medium wash load to whiten T-shirts. A paste of hydrogen peroxide and baking soda can be rubbed into severe T-shirt stains with a clean toothbrush and the shirt then washed as usual. Borax is a white-to-clear mineral used as a natural cleaner. Add 1 cup of borax to a small-to-medium wash load to whiten T-shirts. For extremely discoloured T-shirts, soak the shirt in 1 cup of borax mixed with 5 gallons of hot water overnight and then wash as usual.