A Curezone poster is frequently mentioning nitrates and nitrites and how nitrites can cause poisoning through the formation of methemoglobin. Here is a copy of the claim:

'The term "nitrate toxicity" is commonly used but the toxic principle is actually
nitrite. When Nitrate is converted to nitrite. Nitrite is absorbed from the
digestive system converting blood hemoglobin to methemoglobin. Methemoglobin
cannot transport oxygen to body tissues, so People and animals die from oxygen
insufficiency.'

The poster keeps leaving out one very  important point discussed in this link he got his information from:

http://www.ead.anl.gov/pub/doc/nitrate-ite.pdf

"What Happens to It in the Body? Nitrate is a normal component of the human diet, with the average

daily intake from all sources estimated at 75 milligrams (mg), or about 0.0026 ounce. Upon ingestion,

about 5% of the nitrate taken in by healthy adults is converted (reduced) to nitrite by bacteria in saliva.

Further nitrate is converted by bacteria inside the alimentary tract. Certain conditions in the stomach can

increase the conversion of nitrate to nitrite, specifically when the pH of the gastric fluid is high enough

(above 5) to favor the growth of nitrate-reducing bacteria. This process is of major concern for infants,

whose gastrointestinal systems normally have a higher pH than those of adults. Nitrites in the stomach

can react with food proteins to form N-nitroso compounds; these compounds can also be produced when

meat containing nitrites or nitrates is cooked, particularly using high heat. While these compounds are

carcinogenic in test animals, evidence is inconclusive regarding their potential to cause cancer (such as

stomach cancer) in humans.

What Are the Primary Health Effects? Nitrates themselves are relatively nontoxic. However, when

swallowed, they are converted to nitrites that can react with hemoglobin in the blood, oxidizing its

divalent iron to the trivalent form and creating methemoglobin. This methemoglobin cannot bind oxygen,

which decreases the capacity of the blood to transport oxygen so less oxygen is transported from the lungs

to the body tissues, thus causing a condition known as methemoglobinemia.

Normal individuals have low levels (0.5 to 2%) of methemoglobin in

their blood. When this level increases to 10%, the skin and lips can take

on a bluish tinge (cyanosis), and levels above 25% can cause weakness

and a rapid pulse. At levels above 50 to 60%, a person can lose

consciousness, go into a coma, and die. Infants are much more sensitive

than adults to nitrates/nitrites, and essentially all deaths from

nitrate/nitrite poisoning have been in infants. Long-term exposure to

lower levels of nitrates and nitrites can cause diuresis (an increase in the

amount of urine, and starchy deposits and hemorrhaging of the spleen)."

Just more evidence showing the dangers of alkalinizing the stomach with alkaline waters or drinks, or using antacids or acid blockers.