First of all, Nitrate and nitrite are considered hazardous, and there are legal limits to their concentration in food and drinking water.
From the Institute of Health Sciences, VU University, Amsterdam, Netherlands:
The World Health Organization (WHO) first set an upper limit for
nitrate in food in 1962 (4). It was based on a brief report from the
US Food and Drug Administration (5), which stated the following:
“sodium nitrate has been fed to rats at levels up to 10% in the diet
for their lifetime. Other than some depression on growth at levels
above 1% of nitrate, no adverse effects were noted in these animals.
Two dogs were fed 2% sodium nitrate in their diet for a period of 105
and 125 days, respectively. No adverse effects were noted” (p 136).
The WHO calculated from this that daily intakes of ≤500 mg of sodium
nitrate/kg body weight were harmless to rats and dogs. This figure was
divided by 100 to yield an Acceptable Daily Intake for humans of 5 mg
sodium nitrate or 3.7 mg nitrate per kg body weight, which equals 222
mg for a 60-kg adult. That figure has stood ever since.
Putting a limit on that indicates a form of danger.
Further there's a mention of what can result when consuming nitrates:
It is undisputed that nitrate ingestion widens arteries. Bacteria in
the mouth and gut reduce nitrate to nitrite, which is then converted
by nitric oxide synthase into the endothelium-derived relaxing factor
nitric oxide (2). That is why sublingual nitrate can resolve an
episode of angina pectoris.
There are some minor other things discussed in that article, but none are supported very well, but nevertheless good to know.
On Scientific American, the intake of nitrate has positive results:
The story of nitrate's positive side began in 1994, when Jon Lundberg
of the Karolinska Institute in Stockholm and Nigel Benjamin of
Peninsula Medical School in Exeter, England, independently observed
that the human stomach harbors large amounts of the gas nitric oxide
(NO). Lundberg and Benjamin immediately suspected that the gas might
be killing germs in the stomach, because nitric oxide, when presented
to microbes by white blood cells, weakens them.
The question was where the gas was coming from. Nitric oxide performs
several vital functions in the body, including dilating blood vessels,
and for these activities, a cellular enzyme called nitric oxide
synthase extracts the gas molecule from arginine, an amino acid.
Chemists have long known another mechanism: at low pH, nitrite (NO2)
forms a stew of nitrogen-oxygen compounds, including nitric oxide.
Bacteria in the mouth convert nitrate to nitrite, which gets
swallowed, so the stomach can naturally produce nitric oxide. If
nitric oxide were truly beneficial to the stomach, harmless bacteria
feeding on nitrate-rich saliva might have a symbiotic relationship
with humans.
Benjamin's group confirmed the antimicrobial effect right away by
exposing bacteria responsible for stomach infections to stomach acid
both alone and mixed with nitrite. Although acid is often thought to
be the stomach's main line of defense against invading bugs, the
researchers found that E. coli, Salmonella and other bacteria could
survive for hours in it, whereas high normal concentrations of nitrite
plus acid killed the bacteria in less than an hour. Next, Lundberg and
his co-workers placed saliva from people who had ingested nitrate
tablets onto the inside surface of the stomachs of rats. The mucous
membranes lining their stomachs thickened and received more blood,
both of which are important barriers to infection and ulcers. Rats
that received nitrate-poor saliva showed no change. Benjamin has also
observed that cavity-causing bacteria self-destruct in a high- nitrite
environment, suggesting an experiment to see if a high-nitrate diet
prevents cavities.
In the article of the University of Nebraska - Lincoln, They also talk about nitrate danger for infants and cancer related problems.
About the methaemoglobine with infants.
In every one of the instances in which cyanosis (the clinical symptom
of methaemoglobinaemia) developed in infants, the wells were situated
near barnyards and pit privies.” There was an absence of
methaemoglobinaemia when formula milk replacements were made with tap
water. Re-evaluation of these original studies indicate that cases of
methaemoglobinaemia always occurred when wells were contaminated with
human or animal excrement and that the well water contained
appreciable numbers of bacteria and high concentrations of nitrate
(Avery, 1999). This strongly suggests that Methaemoglobinaemia,
induced by well water, resulted from the presence of bacteria in the
water rather than nitrate per se. A recent interpretation of these early studies is that
gastroenteritis resulting from bacteria in the well water stimulated
nitric oxide production in the gut and that this reacted with
oxyhaemoglobin in blood, converting it into methaemoglobine.
And secondly, about cancer risks:
About 50 epidemiological studies have been made since 1973 testing the
link between nitrate and stomach cancer incidence and mortality in
humans, including Forman et al. (1985) and National Academy of
Sciences (1981). The Chief Medical Officer in Britain (Acheson, 1985),
the Scientific Committee for Food in Europe (European Union, 1995), and
the Subcommittee on Nitrate and Nitrite in Drinking Water in the USA
(NRC, 1995) all concluded that no convincing link between nitrate and
stomach cancer incidence and mortality had been established.
A study reported by Al-Dabbagh et al. (1986) compared incidence of
cancers between workers in a factory manufacturing nitrate fertilizer
(and exposed to a high intake of nitrate through dust) and workers in
the locality with comparable jobs but without the exposure to
nitrate. There was no significant difference in cancer incidence
between the two groups.
Based on the above findings showing no clear association between
nitrate in drinking water and the two main health issues with which
it has been linked, some scientists suggest that there is now
sufficient evidence for increasing the permitted concentration of
nitrate in drinking water without increasing risks to human health.
