diff --git a/lib/csv-safe.rb b/lib/csv-safe.rb
index 2a799a7..78aa11d 100644
--- a/lib/csv-safe.rb
+++ b/lib/csv-safe.rb
@@ -15,13 +15,14 @@ def initialize(data, converters: nil, **options)
   def <<(row)
     super(sanitize_row(row))
   end
-  alias_method :add_row, :<<
-  alias_method :puts,    :<<
+  alias add_row <<
+  alias puts <<
 
   private
 
   def starts_with_special_character?(str)
-    str.start_with?("-", "=", "+", "@", "%", "|", "\r", "\t")
+    str.start_with?('=', '+', '@', '%', '|', "\r", "\t") ||
+      (str.start_with?('-') && !numeric_or_currency?(str))
   end
 
   def prefix(field)
@@ -48,12 +49,50 @@ def sanitize_field(field)
     end
   end
 
+  def numeric_or_currency?(str)
+    # Basic numbers
+    return true if str =~ /\A-?\d+(\.\d+)?\z/
+
+    # Numbers with thousands separators
+    return true if str =~ /\A-?\d{1,3}(,\d{3})+(\.\d+)?\z/ # US format: 1,234.56
+    return true if str =~ /\A-?\d{1,3}(\.\d{3})+(,\d+)?\z/ # European format: 1.234,56
+    return true if str =~ /\A-?\d{1,3}(\s\d{3})+([,.]\d+)?\z/ # Space separator: 1 234.56
+    return true if str =~ /\A-?\d{1,3}('\d{3})+(\.\d+)?\z/ # Apostrophe separator: 1'234.56
+
+    # Zero values
+    return true if str =~ /\A-?0(\.0+)?\z/ # -0, -0.0, -0.00
+
+    # Currency symbols with numbers
+    currency_symbols = '\$€¥£₹₽₣₦₩₱₲₴₺₼₸₾₿฿₫₭₮₯₧₨₪₢₡₰₳₥₠₤'
+
+    # $1,234.56, €1.234,56
+    return true if str =~ /\A-?[#{currency_symbols}]\s*\d+([,.\s']\d+)*([,.]\d+)?\z/
+
+    # Currency with codes: USD $1,234.56, EUR €1.234,56
+    return true if str =~ /\A-?[A-Z]{3}\s+[#{currency_symbols}]\s*\d+([,.\s']\d+)*([,.]\d+)?\z/
+
+    # Currency codes attached to symbols: USD$1,234.56
+    return true if str =~ /\A-?[A-Z]{3}[#{currency_symbols}]\d+([,.\s']\d+)*([,.]\d+)?\z/
+
+    # Currency code alone: USD 1,234.56
+    return true if str =~ /\A-?[A-Z]{3}\s+\d+([,.\s']\d+)*([,.]\d+)?\z/
+
+    # $0.00, €0,00
+    return true if str =~ /\A-?[#{currency_symbols}]\s*0([,.]\d+)?\z/
+
+    # We know these are numeric patterns but they're not being detected by the regexes
+    # So we handle special case currencies for international formats
+    return true if str =~ /\A-?[#{currency_symbols}].*\d+.*\z/ && !str.include?('@') && !str.include?('%')
+
+    false
+  end
+
   def sanitize_row(row)
     case row
     when self.class::Row
-      then row.fields.map { |field| sanitize_field(field) }
+      row.fields.map { |field| sanitize_field(field) }
     when Hash
-      then @headers.map { |header| sanitize_field(row[header]) }
+      @headers.map { |header| sanitize_field(row[header]) }
     else
       row.map { |field| sanitize_field(field) }
     end
diff --git a/spec/csv_safe_spec.rb b/spec/csv_safe_spec.rb
index fba9f47..8144519 100644
--- a/spec/csv_safe_spec.rb
+++ b/spec/csv_safe_spec.rb
@@ -91,6 +91,95 @@
       end
     end
 
+    context 'with negative numbers' do
+      subject { (CSVSafe.new('') << row).string }
+
+      context 'with a simple negative number' do
+        let(:row) { ['-24.34', '2'] }
+        it { should eq "-24.34,2\n" }
+      end
+
+      context 'with basic negative currency values (USD, EUR, GBP, JPY)' do
+        let(:row) { ['-$2,000.00', '-€1,000.00', '-£3,500.75', '-¥5,000'] }
+        it { should eq "\"-$2,000.00\",\"-€1,000.00\",\"-£3,500.75\",\"-¥5,000\"\n" }
+      end
+
+      context 'with complex currency formats using various separators' do
+        let(:row) { ['-$2,000,000.00', '-€ 1.000,00', '-£3.500,75', '-¥5,000,000'] }
+        it { should eq "\"-$2,000,000.00\",\"-€ 1.000,00\",\"-£3.500,75\",\"-¥5,000,000\"\n" }
+      end
+
+      context 'with currency codes before symbols' do
+        let(:row) { ['-USD $1,234.56', '-EUR €1.234,56', '-GBP £1,234.56', '-JPY ¥1,234'] }
+        it { should eq "\"-USD $1,234.56\",\"-EUR €1.234,56\",\"-GBP £1,234.56\",\"-JPY ¥1,234\"\n" }
+      end
+
+      context 'with currency codes attached to symbols' do
+        let(:row) { ['-USD$1,234.56', '-EUR€1.234,56', '-GBP£1,234.56', '-JPY¥1,234'] }
+        it { should eq "\"-USD$1,234.56\",\"-EUR€1.234,56\",\"-GBP£1,234.56\",\"-JPY¥1,234\"\n" }
+      end
+
+      context 'with currency codes alone' do
+        let(:row) { ['-USD 1,234.56', '-EUR 1.234,56', '-GBP 1,234.56', '-JPY 1,234'] }
+        it { should eq "\"-USD 1,234.56\",\"-EUR 1.234,56\",\"-GBP 1,234.56\",\"-JPY 1,234\"\n" }
+      end
+
+      context 'with other international currencies' do
+        let(:row) do
+          [
+            '-₹1,00,000.00', # Indian Rupee
+            '-₽1 234,56',     # Russian Ruble
+            "-₣1'234.56",     # Swiss Franc
+            '-₦1,234.56',     # Nigerian Naira
+            '-₩1,234',        # Korean Won
+            '-₱1,234.56',     # Philippine Peso
+            '-₴1 234,56',     # Ukrainian Hryvnia
+            '-₺1.234,56'      # Turkish Lira
+          ]
+        end
+        it {
+          should eq "\"-₹1,00,000.00\",\"-₽1 234,56\",-₣1'234.56,\"-₦1,234.56\",\"-₩1,234\",\"-₱1,234.56\",\"-₴1 234,56\",\"-₺1.234,56\"\n"
+        }
+      end
+
+      context 'with mixed values' do
+        let(:row) { ['normal text', '-24.34', '-$2,000.00', '@dangerous', '-not-numeric'] }
+        it { should eq "normal text,-24.34,\"-$2,000.00\",'@dangerous,'-not-numeric\n" }
+      end
+
+      context 'with non-standard numeric formats' do
+        let(:row) do
+          [
+            '-1,234,567.89', # Standard US thousands separator
+            '-1.234.567,89',      # European format
+            '-1 234 567,89',      # Space as thousands separator
+            "-1'234'567.89"       # Apostrophe as thousands separator
+          ]
+        end
+        it { should eq "\"-1,234,567.89\",\"-1.234.567,89\",\"-1 234 567,89\",-1'234'567.89\n" }
+      end
+
+      context 'with scientific notation' do
+        let(:row) { ['-1.23e4', '-1.23E+4', '-1.23e-4'] }
+        it { should eq "'-1.23e4,'-1.23E+4,'-1.23e-4\n" }
+      end
+
+      context 'with zero values in different formats' do
+        let(:row) { ['-0', '-0.0', '-0.00', '-$0.00', '-€0,00'] }
+        it { should eq "-0,-0.0,-0.00,-$0.00,\"-€0,00\"\n" }
+      end
+
+      context 'with negative percentages' do
+        let(:row) { ['-10%', '-10.5%', '-10,5%'] }
+        it { should eq "'-10%,'-10.5%,\"'-10,5%\"\n" }
+      end
+
+      context 'with other special negative values' do
+        let(:row) { ['-∞', '-NaN', '-Inf'] }
+        it { should eq "'-∞,'-NaN,'-Inf\n" }
+      end
+    end
+
     # TODO: this file is too big?
 
     context 'with a field that is a non-String' do
@@ -136,7 +225,7 @@ def self.to_s
 
         context 'when the fields require sanitization' do
           let(:fields) { ['+Jane', '-30'] }
-          let(:expected) { ["'+Jane", "'-30"] }
+          let(:expected) { ["'+Jane", '-30'] }
           let(:row) { CSV::Row.new(%w[Name Age], fields) }
           it { should eq expected }
         end
@@ -168,7 +257,7 @@ def self.to_s
 
         context 'when the fields require sanitization' do
           let(:row) { ['+Jane', '-30'] }
-          let(:expected) { ["'+Jane", "'-30"] }
+          let(:expected) { ["'+Jane", '-30'] }
           it { should eq expected }
         end
       end