A question: We know getting cut on a rusty item can cause Tetanus. But some old drinking water pipes will be full of rust. How is it we can drink tap water that comes from these pipes with no problem but at the same time rust can cause Tetanus?

Answer to the above Q: Rust is a part of life. Anywhere there is metal exposed to the elements, there will likely be rust and flaking metal. Rust is the result of a complex chemical reaction involving iron, water and oxygen from the air. This compound is a result of iron (Fe) atoms combining with oxygen (O), and it has the chemical formula of Fe2O3. Its formal name is iron (III) oxide. The (III) in its name shows that the iron atom in this molecule has lost three electrons to the oxygen atoms. 

Whether you have old rusty pipes in your home or a desire to buff the rust off of your vehicle, if you plan on spending any time around this type of iron corrosion, rust ingestion can occur.

Rust particulates can be irritating to the eyes, like any dust. Ferric oxide can also cause an upset stomach, but only if you ingest it in large quantities. The main hazard of ferric oxide is inhaling it as a fine dust or fumes. Inhalation causes lung irritation and coughing. Long-term inhalation causes a condition known as siderosis where iron is deposited in the lungs, although this condition is normally considered benign and does not necessarily lead to physical problems.

Small amounts of rust in your system is not dangerous like you get through your water pipes provided the pipes are clean.

Trace amounts of iron is necessary in order for oxygen to be distributed throughout the body — Iron is a mineral found in every cell of the body. Iron is considered an essential mineral because it is needed to make hemoglobin, a part of blood cells. 

Rust is made out of iron(III) oxides (termed ferric oxides) which is a different oxidation state of iron to that found in haemoglobin (iron(II) - termed ferrous.

But ingesting too much rust can lead to health issues...

Again this depends on your exposure. If you stepped on a rusty nail, the  bacteria Clostridium tetani, found in the dust surrounding the nail, in an anaerobic environment (the deep, closed wound) is the problem and causes tetanus, not the iron. Small amounts of iron oxide are likely to be found in picked vegetables (onions, lettuce, etc) that have small amounts of dirt. Your body will essentially ignore this because the iron is Fe(III), which has the wrong number of electrons (and is generally insoluble at physiological conditions). Your body wants Fe(II), which is soluble, mainly for incorporation into heme in red blood cells. 

However, consuming large quantities of rust contaminated well water  can cause serious health issues including diarrhea, stomach ache, nausea and vomiting. In severe cases, liver failure and major cardiovascular complications may occur characterized by rapid breathing, increased heart rate and weak, erratic pulse.

There is also the possibility of acute iron toxicity  when consumed in the Fe (II) state . Iron is a powerful oxidizer reducing agent and a child who finds iron pills can eat them all and promptly create very corrosive conditions in his gut. Assuming the gut stays intact long enough, the iron can go to the liver where it can profoundly interfere with mitochondrial function and the liver cells can become dysfunctional or die. Don't forget that this is all from Fe(II), not the Fe(III) found in rust.

Having ferric iron oxide as cofactors for haemoglobin is actually quite a dangerous disease called methemoglobinemia, as the altered oxidation state of ferric iron gives haemoglobin a reduced affinity for unbound oxygen and increased affinity for bound oxygen, which makes oxygen delivery to tissue less efficient.

It should be difficult/impossible to get methemoglobinemia from ingesting rust from corroded pipes but you're wrong to assume it's safe because haeme ( deep red, iron-containing compound ) is what our blood is made of. Oxidation states are very important. Fe(III ) or Fe3+ (ferric oxidation state),  can become Fe (II ) or Fe2+ (ferrous oxidation state )  by accepting an electron.

A genetic disorder called hemochromatosis affects the body's ability to control how much iron is absorbed. This leads to too much iron in the body. Treatment consists of a low-iron diet, no iron supplements, and phlebotomy (blood removal) on a regular basis.

It is unlikely that a person would take too much iron through normal food and water.

Fe(III) is quite bad for you if you get it (or make it) in your blood cells, but the gut is essentially uninterested in it to begin with. Now, there are ways to make that chemistry happen but, eating or drinking rust through water isn't, generally, one of them.

Excess iron is readily eliminated from the body, and most people can actually benefit from additional dietary iron. Only consuming it in large quantities will make you sick.

Adults with normal intestinal function have very little risk of iron overload from dietary sources of iron.
And how much of iron is safe?

The FDA of the US suggests that iron intake of 8 mg/day for men, and 18 mg/day for women. Iron toxicity occurs at about 45 mg/day.

Here is the full list of iron requirements of people of different age  groups...

The Food and Nutrition Board of the US at the Institute of Medicine recommends the following:

Infants and children

• Younger than 6 months: 0.27 milligrams per day (mg/day)*
• 7 months to 1 year: 11 mg/day
• 1 to 3 years: 7 mg/day
• 4 to 8 years: 10 mg/day

*AI or Adequate Intake

Males

• 9 to 13 years: 8 mg/day
• 14 to 18 years: 11 mg/day
• Age 19 and older: 8 mg/day

Females

• 9 to 13 years: 8 mg/day
• 14 to 18 years: 15 mg/day
• 19 to 50 years: 18 mg/day
• 51 and older: 8 mg/day
• Pregnant women of all ages: 27 mg/day
• Lactating women 19 to 30 years: 9 mg/day

Women who are pregnant or producing breast milk may need different amounts of iron.

A warning: Water leaking from corroded pipes you use in your house can destroy the iron frame of your building making it weak. So changing the pipes is better as soon as you notice the degradation.

Moreover, if the rusted and worn out iron water pipes are nearer to sewerage (sewage) pipes, drinking water might get contaminated. Therefore, see that these two pipe lines are placed at different areas of the building or compound of your house. Once they become old and get damaged, it is better to change them.